The issue of paintings in Sims 3 is a lot more complicated than it was in Sims 2. In Sims 2, all of your Sims painted the same paintings, and the paintings varied by skill level. For the most part, the sequence of paintings was fairly predictable.
In Sims 3, paintings vary from one Sim to another, along with their skill level. According to some early press releases, the paintings your Sims produced could also be things that they had seen. I remember a quote somewhere about how two Sims could go to the beach, come home and make paintings, and each painting would be slightly different, depending on the Sim's interpretation.
My Sim Ana has the "artistic" trait, so I assume she is learning to paint faster than other Sims would. Her other traits are computer whiz, evil, tidy, and no sense of humor. Her favorite food is sushi, her favorite music is indie, her favorite color iss orange, and her lifetime wish is to have five children. All that being said, here's a retrospective of the painting she's made so far. (Click for big.)
1. This green swoopy thing was her first painting. I have no idea if it's supposed to represent something or not. She sold this painting for astonishing $75.00.
2. Her second painting was quite a departure from the first. I wonder if this has something to do with her lifetime want to have five children? This painting also sold for $75.00.
3. Her third painting cracked me up. You may recognize this as one of the tile patterns from the first and second Sims games. I have to admit, she does enjoy cleaning the bathroom, and I wonder if this painting is an expression of that?
I have yet to find any definitive answer on whether or not you can drop in JPGs and have your Sims create them as custom paintings. I did find one web site which published this as a Sims 3 tip, but they screwed up in several places and referred to Sims 2, so I'm guessing it was just a copy and paste job.
I did learn that your Sims can create portraits and custom paintings once they reach level five. However, this is not the same process as it was in Sims 2. Check out this LiveJournal entry by Sims 3 player sarianastar, who experimented with the Still Life paintings. Whereas in Sims 2 a custom painting would be photorealistic, her Sims painted interpretive, expressive versions of what she captured with the screen shot.
I wonder if this has to do with the skill of the sim doing the custom painting, if a sim with more painting skill would make a more realistic painting? Or if this is just an example of her Sims expressing themselves by painting interpretive still lifes, with a little bit of artistic flair.
I definitely want to test out the early theory, that two Sims could each watch a sunset and then make a different painting out of it. I'll keep you posted!