Depends. Most commonly they sell their souls to the devil in exchange of art skills. Some artists have confessed that they are infact aliens that study how humans respond to images out of the ordinary. In some occasions , an individual will have the luck to move to a house that has a hidden crypt that was build by occult wizards in ancient times. The crypt will provide a new piece of art every sunday for the individual to grab and show as his.
In my case , ten years ago i was climbing in a tree to catch a parrot. As i did i accidentally slipped in squirel droppings. After a long dramatic fall i landed on my head. When i woke up , 4 years ago , i realised in astonishment that i could make artz but could no longer play the piano.