My two Sims were a painter and a comedian. Like some others, I made them vampires, so they wouldn't spend much time on needs. The comedian bred frogs to convert them to plasma packs. For hygiene, fun and social, they used the crystal crown.
I built them a tiny home with a large roofed porch, so they could be outside during the day. They had a wedding arch and constant holidays to keep them happy. They also had clubs running. In the beginning this was useful for the skill bonus, and then they formed the audience for the comedian (which wasn't all that important).
My painter collected enough aspiration points to pick up marketable and creative visionary (with the beginning of soulmate, vampire aspirations, and painter extraordinaire). She used the digital sketchpad to make abstract paintings (which have the highest average value).
I made a spreadsheet to figure out how to sell the paintings. You can sell them at the flea market canvas thing at 4 times the original price, but that does mean there is less time for painting. It takes about an hour to sell a painting on average (but this does vary a lot). Initially, this means it is worth it from roughly § 2500. All paintings with lower value, my Sim sold directly, while wearing the crystal crown with diamond to gain fame.
Now, with fame rank 3 there is a perk that makes you make more money from skills. It actually took me a little while to figure out how this works. The value you see when you hover over the painting or sell it through inventory stays the same. But if you sell it in the world, from a wall or easel, you get 1,5 times more money. This means the cut-off point for having flea markets also changed, to ca. § 4000. A painting with this value would give § 6000 selling directly, or § 16000 in a flea market.
The highest value I got from a single painting was § 52,168 (in a flea market sale).
My comedian meanwhile, while gaining fame faster than my painter, was making enough money to support himself, but nothing meaningful for this challenge. Still, it was nice to have some cash coming in during slow flea markets.
I know camping mascots are also valuable, but I thought painting was more lucrative. I didn't actually check, though. Now I'm wondering.