This was written by the art director of the Battle for Wesnoth project (i.e. Richard “Jetryl” Kettering) and it explains better than I could why Wesnoth uses the GPL for its artwork (well, more generally, why it uses licenses without no-commercial-use, no-derivations, attribution, or similar clauses, in the context of a discussion about whether to switch from the GPL). It’s kind of long, but I think definitely worth reading.
I’m just linking rather than quoting it here because getting the formatting to work right would be a real pain, and isn’t worth it. I know I usually don’t just give links, but this is something I do think you should read, especially if you’re somewhat indifferent about the whole idea of open-source artwork.