I have been looking through the wiki, rules, guidelines and forums to find instructions to a specific problem, but I haven't found a direct answer:
When an artist takes down all their art, you are not allowed to re-upload it. That's easy. Everybody should respect the creator's wishes. Now to the problem that needs a solution (as far as I can tell). What about the commissioner of a certain piece of art? Is it allowed for them to re-upload the picture they have paid for if they choose to share it? Even against the artist's wishes?