Finally, I found here the answer I have been looking for all this time:
http://www.gnu.org/distros/free-system-distribution-guidelines.en.html
It seems that the license of the game (for instace, a GPL one) does not affect to the art (sprites, music, etc), which is considered as non-functional data. So that using the CC license for the art would be fine, as far as I understand.
http://www.gnu.org/distros/free-system-distribution-guidelines.en.html
It seems that the license of the game (for instace, a GPL one) does not affect to the art (sprites, music, etc), which is considered as non-functional data. So that using the CC license for the art would be fine, as far as I understand.