There are 10 levels in the painting skill. Painting is one of the easiest ways to make money in the Sims 4. Each painting can be sold or kept. This skill goes along with the painting career, so you can max out both the skill and job in one go. The lot trait Natural Light will give the sim a +50% skill gain boost and the lot trait Home Studio, will help the sim make higher quality paintings at a lower level.
To get started either buy an easel from the buy mode or purchase the painting skill books from a bookshelf.
I found the easiest way to get this skill maxed out is just to have a jobless sim do nothing but paint in their free time. It helps fund the household while also getting an achievement done.