The GPL doesn't really apply to graphics, I mean how could you show the source code?
There are other licenses, but all licenses are really just codified agreements of use. So, the DC Uatu license (
) is a picture may only be modified if a black background is replaced with another. All else is not allowed. [Some stuff about not being able to claim as your own, not make money off of Uatu's work without his explicit permission, etc. and/or whatever]
Okay, I'm looking at the nupperibo and I like the addtion of the background, but (I know, "what do you mean but?") because the background is brighter it seems like the bluish shadow along the belly on the lower right hand side, seems it should be darker. (That's all, I promise.) It must be some trick of the color combo.