Teleport yourself to the fort (near the tent with the necromancer npc) and go inside with the key. Use Lohse (Telekinesis 1) to pick up the portrait and put it in your inventory. The 150 weight stuffs sells for like, 80-90 gold. You can also pick up Lucian Portrait at the Shinre to the Seven and sell that as well for another 80-90 gold. You can make pretty much 500-600 gold to recycle around with pickpocket from this method.
THe one you want to sell it to first are the Griff NPCs because you can end up killing them and take them back again to sell once more.
All those paintings are completely unguarded too. No guards around, no patrolling guards who could catch you. They aren't even considered owned by the Magister faction so you can sell them to prison-area magisters!
Most of the ways out of Fort Joy go right through the fort, which means easy access to the paintings. Larian might as well have placed a huge heap of 500 gold in the middle of an empty room for all the balance the paintings have.
This is not a good way to handle the game economy. Every single painting on Fort Joy should be changed to be worth exactly 0 gold, because absolutely no one on the island has any use for such paintings.
