Creative Commons produce a number of licenses. Not all of them are copyleft licenses, indeed only CC-BY-SA (the Attribution-ShareAlike) comes close to the provisions of the GPL. Creative Commons also produce licenses for sampling, file sharing and licensing work to developing nations. Altogether there are around 15 Creative Commons licenses for artists to choose from.
Many artists have licensed images online CC-BY-NC or CC-BY-NC-ND. The problem is that these are non-commercial licenses, which means that other artists cannot use that work to make work that they can sell.
CC-BY-SA allows anyone to copy or use the licensed work. Anyone can print or sell copies of it, and anyone can make new work that uses all or some of the original. This is a good match to the “Four Freedoms” of the GPL, and allows artists access to a broader culture of images. No “Joywar ” under CC-BY-SA. You lose more control of your work – anyone can copy or sell it without paying you, and in return you get access to any work they derive from it and attribution for the use of your work. But however worthwhile GPL-style Freedom is, and it is, it may not be a good match to the existing social contract of art.
CC-Sampling is designed for music but can be applied to any medium. Like CC-BY-SA you can make work that uses part or all of the licensed work. Unlike CC-BY-SA, you must creatively transform the work that you use, you cannot exploit it as-is. You also cannot use use it in advertising. This requirement of creativity and opposition to corporate exploitation is intended to capture the social contract of musicians for sampling and mash-ups, but fits artists as well. This is a much better fit for artists, and protects work against simple exploitation, but may preclude some creative or beneficial uses of the work and may allow the access to less work than the main CC licenses.
To align work with the broader Free Software and Free Culture movements, CC-BY-SA is best, although it is important that artists understand what they are giving away and receiving in return under this license. To match the existing social contract of artists creatively using the work of others, CC-Sampling is best, although it may be limiting in other ways. Artists may balk at placing work under either license. But like Willem De Kooning faced with a young Robert Raushenberg asking for a drawing to erase, they should rise to the challenge and see what happens when they let other people use at least one work to create something unexpectedly new.
(I am not a lawyer, this is not legal advice.)