Eh, I see paintings being valuable to everybody, including prisoners. First off, art in this world would be extremely rare and valuable in general. A beautiful painting in a medieval world would be quite a feast, especially to a prisoner, if anything, where they have so much ugliness in their life. The prisoners or seekers would recognize the items as valuable, something worth keeping onto to sell if they were ever to escape. Obviously the extra large painting is a bit comical to sell to some random person, but at a certain point, it's a game.
But they should be marked as stolen, really, so you can't sell them to the magisters at least.
