source of an image is the what original author gives you (IMO)
(ie defined by its initial author)
so even if he used some layered image to create the final image
if he releases the final ( flattened ) image under GPL He/She doesnt need to give its source
because the final image will be the source.
but there is a problem if he/she uses commercial images that avoid the final product to be released under GPL
I believe that is what happened to the music:
editable or not
It doesn't say anything about how they are created?
Sound samples used in the music don't allow GPL?
There are for instance plenty of layered .psd files on the Internet, for use in tutorials and whatnot. With such a file it doesn't matter whether there is a 'source' (as in multi-layer) or not, the original author can still not be determined without documentation.
Theoretically it would mean taking a random picture from the Internet, adding a layer and then release it myself as GPL work which I have authored. Then who would have the stronger hand in court, the one who actually made the picture but doesn't have it layered and as source, or me who in fact didn't do it, but I have the layers in it and use that as my 'source'?
I don't think there is any way to legally let images act as their own sources, regardless of layers, or not to avoid that piece of license, a document would still be needed.
GPL license requires the source to be available for modification and both a layered, and flattened image, are modifiable. Same with sounds, it's just a matter of how easy it is to modify them.
It is very diffuse with both images and sound since it not possible to include a copy of the GNU license with every image or sound-file, but if you don't; how can you then say where the media belongs? And again, even if you do, how can you prove it's your work? Only chance is with the original tracker for music, layered image for pictures, but how do you then go about when releasing a .bmp as GPL for instance?
3. You may copy and distribute the Program (or a work based on it, under Section 2) in object code or executable form under the terms of Sections 1 and 2 above provided that you also do one of the following:
a) Accompany it with the complete corresponding machine-readable source code, which must be distributed under the terms of Sections 1 and 2 above on a medium customarily used for software interchange; or,
b) Accompany it with a written offer, valid for at least three years, to give any third party, for a charge no more than your cost of physically performing source distribution, a complete machine-readable copy of the corresponding source code, to be distributed under the terms of Sections 1 and 2 above on a medium customarily used for software interchange; or,
c) Accompany it with the information you received as to the offer to distribute corresponding source code. (This alternative is allowed only for noncommercial distribution and only if you received the program in object code or executable form with such an offer, in accord with Subsection b above.)
On of the real problems is most likely that GPL isn't really embracing the use of images/sound.