Ok, I just got a direct update by email from the original author (so, from http://pr09studio.deviantart.com/).
In short, he allows us to use his work, and he will relicense it under either a CC-By-SA license, either an equivalent one. What we will do then, as Anne explained before: - rebuild ISOs with updated CREDITS file and graphic elements derivative from the original background; we're still discussing whether we keep the current one or the source one; - recommit in SVN, nonetheless, all graphic elements to log the credits to the original author; - post a blog note about this; - rethink how we manage future contributions in artwork.
