Later in The Voyage of the Dawntreader, Aslan appears to Caspian on board the Dawntreader:

...

But when the others rejoined him a little later they found him changed; he was white and there were tears in his eyes.
 
"It's no good," he said. "I might as well have behaved decently for all the good I did with my temper and swagger. Aslan has spoken to me. No — I don't mean he was actually here. He wouldn't fit into the cabin, for one thing. But that gold lion's head on the wall came to life and spoke to me. ...

In this scene, Aslan is able to appear through a golden (I assume sculpture of a) lion's head.  Given that the Narnian's know Aslan, the golden statue was likely sculpted based on Aslan, himself.  Nevertheless, Aslan is able to use this ordinary thing to manifest himself to Caspian.  Thus, I think Aslan could have used a painting of a random boat and morphed it into the Dawntreader if he had wanted to.

On the other hand, Aslan exists in our world too under another name.  This may belong on the theology thread, but people have debated for centuries over predestination vs. foreknowledge or free-will (or some combination thereof) in their understanding of God.  Aslan could on the one hand have known beforehand what the Dawntreader would look like and inspired a painter on earth to paint it, or he could have predestined the Dawntreader to look like the ship painted on Aunt Alberta's wall.

Sorry for the boring, wishy-washy answer, but I think pretty much any possibility works; you can just go with your head canon 😀 .

It's an intriguing subject. Either it was entirely random, and Aslan can use anything (a garden gate in a boarding school, or an outdoor country railway platform), or there is indeed a history with a Narnian connection. 

When Lewis wrote the book, he had not conceived the idea of Narnia's creation. But from our viewpoint, we might guess that somehow Prof Digory Kirke or Miss Polly Plummer had been a wedding guest and bought the painting for them. (Even further stretching it, was painting Polly's hobby? And did she have a secret interest in old sailing ships?)

I loved the idea of a painting as the entrance into Narnia. I have always wondered where  C. S. Lewis got the idea.  Could he have visited an art museum with a picture of a ship?  He must have liked nautical adventures which gave him the concept for Voyage of the Dawn Treader.  The idea of the picture worked perfectly for the story since it already had a magical quality about it.  🙂

I love the ideas and this thread.  I don't really have any really original suggestions as to why it could have been right now however.  There are two things this topic makes me think of though - the first being, that this topic presents a great opportunity for fan-fiction (which I'd imagine many on here have read a few of in explaining this mystery), or just fan-imagination/fan-speculation which of course is enjoyable enough.

The other thing this topic reminds me of is that I seem to recall many years back on the old Into the Wardrobe forums, a few of the members did some role-playing in one thread, I seem to recall, someone being an older aged Susan Pevensie and I seem to recall the topic of the picture in the spare room at Aunt Alberta and Uncle Harold's house being raised in that thread.  I seem to recall having enjoyed the fan-fiction role-playing thread so much at the time that I printed off sections of, if not the whole thread.  There's a small possibility (though highly unlikely) it may still be in a special box of mine somewhere - I'll have to take a look though.

Posted by: @narnian78

I loved the idea of a painting as the entrance into Narnia. I have always wondered where C. S. Lewis got the idea.  Could he have visited an art museum with a picture of a ship?  He must have liked nautical adventures which gave him the concept for Voyage of the Dawn Treader.  The idea of the picture worked perfectly for the story since it already had a magical quality about it.  🙂 

Probably, he did, but, though dragon boats & dragon boat racing have been around for 2000 years, according to Wikipedia, a similar picture of a dragon boat or a sailing ship that looked like a dragon boat, wouldn't be really the sort of sailing ship picture that I think many people in UK would have had in mind if they wanted a picture of a sailing ship in 1942. Something like the Fighting Téméraire, or the Battle of Trafalgar picture at the bottom of my comment, a reminder of past victories to cheer themselves up, sounds more like it. Also, before World War II, though Chinese culture would seem exotic, China, itself, was torn by warlords until the Sino-Japanese war of 1937, which in 1941 morphed into the Pacific phase of WW2. And given Eustace's proneness to sea-sickness, perhaps Alberta might have felt the same looking at this picture.  

I've seen umpteen pictures of sailing ships, usually of the traditional kind, & have one, myself, somewhere. But it is just an ordinary sailing ship, though I wouldn't have minded a picture of say, the Endeavour, which at least got Captain Cook around the world, the first time. 

So, to find a picture of a dragon ship, in particular, a definitely Chinese concept, anywhere, even in an ordinary Cambridge house, would have been somewhat unique for C.S. Lewis to see, I should imagine. We went to see the Dawn Treader props at Movie World at Coomera in January, 2011. There is a lovely photo of my husband and myself beside the Dragon head prow, dismantled while it was in storage. Now that is a photo I wouldn't mind hanging in my spare room.  

The Dawn Treader from Walden's version of VDT.

Battle of Trafalgar. 21 October, 1805. 

@waggawerewolf27 

I wonder if there were any lighthouses in Narnia. I don’t remember them being mentioned in Voyage of the Dawn Treader or the other Narnia books, but I am assuming that there were navigational beacons of some kind since there were sailing ships. Would a picture of a lighthouse have worked as well as of a sailing ship?  It might have given that people could have been transported to Narnia through it. My guess is that medieval towers with lights inside them could have helped to navigate ships. Lighthouses in our world have existed since ancient times, e.g. Pharos of ancient Egypt.  They could have been built in Narnia too.  🙂

@narnian78 Would a picture of a lighthouse have worked as well as of a sailing ship?

Simple answer is no, because although Narnia had shipping both before the White Witch, & then again during the Pevensie years, under the Telmarines, they had let the coastal areas become wooded & it was only the 7 Missing Lords who dared take a ship anywhere. That was in Prince Caspian. In VDT, King Caspian visited Galma, Terebithia, & the Lone Islands, where Calormene merchants were buying slaves. By SC, King Caspian was able to take ship from Cair Paravel on the coast, & to return there, at the end. Likely, there were lighthouses in Calormen, 

In the Pevensie Golden Age they had the Splendour Hyaline which is described on a Narnian Wiki as follows.

The ship had a carved swan's head at her prow, with huge wings reaching back almost to her waist. She was also decorated with silken sails and great stern lanterns. Nevertheless, she was also fast and equipped for battle, so that she could have out-run or sunk any ship owned by contemporary Calormen. 

I notice that the Dawn Treader likely followed the same design principles, using a dragon instead of a swan. That is why Lucy said the picture on the wall in her small bedroom at Eustace's place, was so very Narnian. But it was not the sort of detail which would bother Alberta, Eustace's mother, all that much, I should imagine.

@waggawerewolf27 

I live in Michigan, which is known here in the U. S. as the lighthouse state because there are over 100 lighthouses on the shores of the Great Lakes. I could easily picture abandoned lighthouses in Prince Caspian appearing much like Cair Paravel in that story. Although they are not mentioned among the abandoned buildings there could have been lighthouses that were under the trees and elsewhere where the forests had grown up. Nautical buildings are usually designed to last a very long time. I think either a lighthouse or a sailing ship would have worked as a picture in a bedroom in Dawn Treader since they both have a magical quality that C. S. Lewis could have used in his story. Lewis chose a sailing ship, but he could certainly have used a lighthouse because it was near the sea and a picture of it would be interesting enough for his readers.  If the lighthouse was abandoned that would have added a vintage feel to the story and made it more old fashioned and intriguing. 🙂

Posted by: @coracle

But from our viewpoint, we might guess that somehow Prof Digory Kirke or Miss Polly Plummer had been a wedding guest and bought the painting for them. (Even further stretching it, was painting Polly's hobby? And did she have a secret interest in old sailing ships?)

Ooh, I hadn't thought of that! It's certainly an idea that could clear up some of the confusion. (Of course MN being written after VDT would explain why he wouldn't specifically say Digory or Polly at the time.) I can see Alberta not wanting to offend a kindly Polly.

Interesting thoughts, all! And I really like the idea of a lighthouse(s) in Narnia and how that could potentially call people into the land :).

@coracle  (And did she have a secret interest in old sailing ships?)

Didn't she keep a pirate's den up in her parents' attic? Also, Professor Kirke & Polly Plummer might well have been friends of relatives, such as Mr & Mrs Pevensie? After all, Alberta & Harold Scrubb were the parents of one Eustace Clarence Scrubb, who, we are told in no uncertain terms in VDT, was cousin to Edmund & Lucy. 

Pirates weren't just the stereo-typed mob in the Caribbean; they were also in the Mediterranean Sea, often hand-in-glove with the Ottoman Empire, in particular, until the 19th century, along with slave-traders in Africa. Before the 19th century, according to the Perth Maritime Museum, slave-trading was for a long time on the Zanj coast, where ports like Mombasa, Dar-es-Salaam, & Zanzibar were well established by 1500. They could be voyaging in Dhows or Chinese-style junks, or even dragon boats. 

@narnian78 I live in Michigan, which is known here in the U. S. as the lighthouse state because there are over 100 lighthouses on the shores of the Great Lakes. I could easily picture abandoned lighthouses in Prince Caspian appearing much like Cair Paravel in that story.

I expect anything with a door in it might well be a portal to another world, for any fantasy writer. We already have the Tardis for Dr Who fans, why not a lighthouse for someone else, if that takes your fancy? But your abandoned lighthouses are mainly around the Great Lake, aren't they? They aren't on USA's Atlantic coastline, or anywhere near it. 

Not on an ocean coastline like Narnia. Let alone real-life here, for millennia as mysterious as Narnia, or Middle Earth, only partly mapped as late as 1802, & where the first lighthouse ever built in Australia was in 1818 on South Head, to demarcate the entrance to Sydney Harbour, 30 years after the 1st Fleet arrived in 1788. 

The Shipwreck Coast along the Victorian coastline between Cape Otway & Port Fairy, was known for more than 700 shipwrecks, & in 1629, a Dutch ship called the Batavia, was notoriously wrecked on the Houtman Abrolhos (1st discovered in 1619 by Frederick Houtman, a VOC captain in the Dordrecht & the Amsterdam), in a notoriously bloodthirsty incident. There were many other such shipwrecks on the West Coast of Australia, but it wasn't until 1849 that the Wadjemup Lighthouse was built to protect shipping to Fremantle, Perth & the Swan River Colony on Rottnest Island, first discovered in 1696 by Dutch explorer Willem de Vlamingh.

@waggawerewolf27 

The Great Lakes are actually inland seas of fresh water, although the lighthouses serve the same purpose as those on the shores of the ocean.  They tend to be smaller than those on the ocean, but if they perform the same function why wouldn’t a Narnian lighthouse do basically the same thing?  Some are abandoned, but those that are still working use modern technology, which was unknown in Narnia.  Narnia’s lighthouses would be more like lighhouses in Michigan from over a century ago. But they certainly would have something which fit in place in Narnia as much as sailing ships, and they would have made interesting subjects for a painting.  I don’t know for certain what Lewis thought of lighthouses in a story, but I am guessing he would have liked them as much as castles, to which they are often compared. I think they would have worked as well as ships, as I have stated before.  The scenery might be different, but the location doesn’t matter that much since they perform the same purpose. 

@narnian78 The Great Lakes are actually inland seas of fresh water, although the lighthouses serve the same purpose as those on the shores of the ocean.  They tend to be smaller than those on the ocean, but if they perform the same function why wouldn’t a Narnian lighthouse do basically the same thing?

No particular reason why they shouldn't. I suppose if Mrs Beaver can use a sewing machine, why not a lighthouse, if it truly is necessary? After all, there was that lighthouse in Alexandria in Roman times. But I think that going back to medieval times, the series of beacons along the mountainous spine of the South Island of New Zealand used for the Lord of the Rings movies, or something like that, would look far more convincingly authentic for Narnia than any modern-day technology. 

The trouble is that I can't think of where in all of the 7 books is there any real occasion to use a lighthouse. Perhaps when the Splendour Hyaline leaves harbour in Tashbaan in HHB. Or maybe, in Silver Chair, when Caspian casts off on his search for Aslan. 

@waggawerewolf27 

But how would they navigate safely without some kind of beacon? They could light a fire in a medieval tower, but would that be practical in warning ships to stay away from dangerous rocks and shoals? A lighthouse would better serve that purpose since the light would be there all the time. I don’t see where Narnia would be any different from our world in making sailing safer. Aslan could protect the ships by some other method, but a lighthouse like the lamppost seems like something he would approve of, and he would have liked them as a possible way of humans and talking animals helping themselves.

@narnian78 But how would they navigate safely without some kind of beacon? They could light a fire in a medieval tower, but would that be practical in warning ships to stay away from dangerous rocks and shoals?

And you forgot the largest coral reef system in the world, the Great Barrier Reef, following Queensland's coastline from Bundaberg to Cape York Peninsula in the north. Captain Cook, just before 11pm on 11/6/1770, ran aground on that reef in his ship, the flat-bottomed Endeavour, at the later established Cooktown, in the north, & had to go ashore for about 7 weeks to repair it, finally leaving by 5th August.  Lucky for him, the first European to have explored New Holland's east coast, (and to map wherever he was going) & for us Australians, perhaps, the natives, the Guugu Yimithirr people, seemed quite friendly for that time, unlike the Gweagal, at Botany Bay. The Guugu Yimithirr language, was the first Aboriginal language to be recorded, it seems. Cook reached Possession Island on 22/8/1770, using the ship's pinnaces to test for shoals & dangerous rocks as they inched forward towards open sea in the Torres Strait, itself, on their way to the then VOC headquarters at Batavia, an ancient city, now Jakarta, on the then Dutch East Indies island of Java, arriving 10/10/1770.

I could also mention the Dutch captains who explored the west of Australia, such as Abel Tasman, Dirk Hartog, Frederick Houtman, or Willem De Vlamingh, himself searching for the remains of another shipwreck, when he found Rottnest Island in 1696. All without benefit of lighthouses, or any such navigational luxury. I don't suppose that lighthouses necessarily dotted the coastlines of either North or South America in days gone by, either, though surely, ancient civilizations in India, China, Japan & elsewhere would have found lighthouses necessary for themselves. Off the West coast of Australia, the Dutch had long ago established a navigational route called the Brouwer Way to enable VOC ships to commute from Cape Town at the Cape of Good Hope to Batavia. But the Commander of the shipwrecked Batavia, Francisco Pelsaert had to somehow sail from Houtman's Abrolhos in 1629 to Batavia, the city to get help for his marooned passengers, because of the storm which blew them onto the rocks & shoals, another epic sea voyage like that of Bligh of the Mutiny of the Bounty.

Now, my "el cheapo" compact paperback version of the Voyage of the Dawn Treader, that I bought with my own money, for personal use, from a travelling bookseller, doesn't have a picture of the Dawn Treader, itself, in it, though I'm sure there is an illustration somewhere around of Eustace, Edmund & Lucy being sucked into that picture in the bedroom. The basic dimensions of the Dawn Treader that Walden used for its production. were based on Cook's ship, the Endeavour, it seemed, when the dragon ship embellishments were cosmetic. I've included a picture of the Endeavour, below. 

Endeavour from Wikipedia article.