It's quite a conundrum... perhaps the strangest thing is that it shows The Dawn Treader in a particular moment in time. Whoever painted it seemed to have glimpsed it personally, and recently. (At least as far as Narnian time goes.)
This is my theory, or "headcanon" at least... I sincerely doubt that Lewis had it in mind, but I rather like to think of it as being the case.
I'll put it in spoiler tags because it references something mentioned in the next chapter:The Duke of Galma had hoped that Caspian would marry his daughter, but Caspian didn't take to her, later saying to Lucy that she "squints and has freckles." I would imagine that, between having her own hopes dashed, her father's disappointment and the mutterings of the townspeople, the duke's daughter would have been pretty unhappy after The Dawn Treader had set sail for other ports. So, one day she took a walk to a very remote part of the island to get away from everyone and think things over. She's nearly at the point where she wants to leave on the next ship that departs from Galma to escape the public embarrassment, but instead on her way home, she accidentally encounters one of those rare chinks or chasms between Narnia and our world and blunders through it, winding up in the British Isles in the 1800s. There's obviously an adjustment period for her, but she takes well to this new world: she marries a lad who thinks her freckles are pretty, becomes a painter of moderate renown (she squints on account of having an artistic eye) and she enjoys a long and happy life. In her old age, she decides to paint The Dawn Treader as she last saw it as a memorial to the man whose disinterest had ultimately caused her to fall into another world and enjoy much happiness there. Sometime after her death, one of her children (who never liked it much personally) gives the painting away as a wedding present to the future Mr. and Mrs. Scrubb. Tada!
I always felt rather bad for the freckled Duke's daughter when Caspian mentioned her, so I wanted her to have a happy ending.
