Here is an interesting article I found on www.gamedev.net/
The first and foremost issue with attempting to release images under the terms of the GPL is the mandatory preamble in the license.
Anything released under the GPL !MUST! contain a notice. Specifically from section 0 of the license:
| "This License applies to any program or other work which contains a notice placed by the copyright holder saying it may be distributed under the terms of this General Public License."|
The typical notice is the 5000 character block of text stating that it is copyrighted, by whom, that permission is granted under the GPL, and where they can obtain a copy of the GPL.
If the original copyright holder failed to provide that notice within the image file (most allow embedding text) then they probably did not release the image under the terms of the GPL. Most often, the people who claim to have released the resources under the GPL have not actually done so -- they merely intended to do so. It is a subtle but very important legal difference.
Submission of images requires that they "must either by public domain or licensed by the copyright holder under a license that allows anyone to reuse the images. The same is true (though worded differently) for audio and other assets.
Lets assume that the copyright holder actually did incorporate the required notices into the works.
The next issue with non-program GPL content is what constitutes the "preferred form of the work for making modifications to it."
What is the "source code", or preferred format, of images? If they were released as lossless PNG or lossless JPG, that could be one of many preferred formats.
Next, assuming the art assets are in the proper source code, you must determine if you are incorporating them into your non-GPL project, or if they are sufficiently separate entities.
All versions of the GPL allow permissions for GPL and non-GPL systems to interoperate as long as they operate under the principles of being "at arms length". If they are separate entities that don't strictly on each other, then they are fine. The LGPL is much more permissive with these permissions, but the GPL itself is quite generous.
There are a few successful programs that do exactly that. They offer up a fully functional GPL product that can interoperate with a separate non-GPL product.
If the images were incorporated as textures to your project, then clearly the viral properties of the GPL would apply. Your game would directly incorporate a GPL licensed work, and therefore your work must be released under the GPL..
There is a common workaround for this. One such is an emulator and software for the emulator, where both exist under different licenses. In this situation, the two entities are separate. You could use a different emulator with the same data, or you could use the same emulator and different data. Since neither is incorporating the other, merely interoperating with it, the two licenses are not at conflict.
For specific examples, someone could use the (free) Blender3D engine and create a (non-free) game that runs on it. You can use a (non-free) game engine and create a (free) game that runs on it.
About a decade ago there was a really competitive instance of this. There was a text-based game engine, I forget what it was called. There was a fairly successful non-gpl engine that was competing with a GPL engine, both able to parse and process the actual game code. Some of the released games were under the terms of the GPL, others were proprietary. There were many questions about if any one of them impacted the other, and the ultimate consensus was that they were two separate programs working together. I wish I could remember the names of those products, and Google isn't helping much.
Ultimately the business answer is simple: If you cannot ensure that the things you are working on are two separate (not incorporated) working independently of each other with completely separate licenses, or if you are unsure about the legal status and cannot otherwise get permission, then don't do it.