That's the same problem that I had with my site:
http://www.imageafter.com/terms.phpThis is the dangerous part:
WHAT CAN'T YOU DO WITH IMAGE*AFTER IMAGES AND TEXTURES?
* you CANNOT REDISTRIBUTE our images and textures as part of an online resource site like our own, i.e. use them to directly compete with us.
With this, a GPL release is impossible! And I don't know if (a low level) creative commons license is possible.
What I did to fix this problem:
I asked the owner of the site (not imageafter! ) to give me permission to release my textures which contain ( in glests case) very small snippets and small versions of the textures and I got permission. You should really try the same when you want to release it finally.
( send them your tgas as jpeg when asking, so they will easily see, that it is no problem for them! )
Thats the reason why i started to collect my own photos to release them as really free ( public domain ).
I found a site where this is possible.
http://www.burningwell.orgI uploaded my first 300 photos/textures now and started to build up the textures section ( I've got some more privileges from the owner of the site)