Usable concept art

Until a better place is found, we can gather concept art in this thread. And I mean real concept art that Xonotic can use to base stuff on. Stuff like this:

http://opengameart.org/content/grey-soldier-concept

and this:

http://opengameart.org/content/assassin-bot

These two are both relased under GPL 2.0, GPL 3.0 and some CCs. I was wondering what kind of license does the concept art need to have to be able to make model based on it and then put the model into Xonotic?

More potential concept art:

http://opengameart.org/browse-menu/images/1073/10