Some more clarifications.
Zarel wrote:You mean dual-licensed GPLv2 + CC-BY-SA-3.0.
By "you mean", I mean that Olrox is a friend of mine, so I can put words in his mouth and he won't mind.

Don't worry, I don't do this to strangers.
Zarel wrote:With a few exceptions, all new contributions need to be licensed one of:
- GPLv2 + CC-BY-SA-3.0
- GPLv2 + CC-BY-3.0
- CC-0
And "need" is the wrong word here. I mean "should". Artwork that is licensed pure GPLv2, or pure CC-BY-SA-3.0, is still valid for inclusion for Warzone. We simply prefer them to be dual-licensed (and there's no reason not to - if you're fine with distributing your artwork under one license, you should be fine with the other).
Zarel wrote:GPL for license compatibility with open-source code, and a CC license because of the legal problems that could occur from the fact that the GPL can't be applied to artwork.
By "license compatibility with source code", I don't mean that pure CC works are incompatible in the "we can't distribute them together" sense, but rather in a "these licenses are incompatible - one cannot be relicensed into the other" sense, as in "we can't just say 'this game is GPL', we have to say 'this game is GPL, except for parts of it which are CC".
- FSF Licensing Compliance Engineer Brett Smith says that there are several problems that prevent users from combining CC-BY-SA and GPL components into a single work. "Perhaps the most straightforward one is that they're both copyleft licenses: each license wants the entire work to be released under its own terms, so no matter what you do you can't satisfy them both." (source)
By "legal problems with GPL on artwork", I mean that the GPL is written in a way that precludes it from being applied to artwork, and forcing it could be interpreted to simply result in an invalid license.
Here is a page that discussion a solution:
http://www.linux.com/archive/feed/119212
"release them in a GPL-licensed and a CC-licensed form."
That is what I recommend all our artists do, because there is no reason to do otherwise.