A new building in Providence (or that big inaccessible building at the NPC market) where you can display your original paintings for a fee and people can buy copies or even the original. The owner of the painting gets 10% of the price of a copy, the rest goes to the crown because it's His Majesty's painters that make the copy; for originals you get 65% (35% is tax, same as in stalls). To get your painting into the game you'd need to send an image file to the devs and they give you an original (it gets a special frame or something to differentiate it from copies). The devs would of course make sure the painting is original and doesn't contain inappropriate content.
Prices should be set by the painting owner. It should be a fixed price for copies. For originals it should be some kind of an automated auction, and the seller sets the initial price.
Possible issue is spamming the devs with frivolous submissions. To address this, you should have to buy an empty canvas to make a submissions, and you don't get the silver back if the painting is rejected.
It'd be cool if it was possible to submit statues as well, if it's feasible.