Spring RTS Engine

FWIW i believe CC is the best licence for non code
http://creativecommons.org/choose/

The most permissive of which is CC-By

The best situation would be pull requests with resources from the respective authors with text licencing in their folder and exif/metadata in the image stating the author and licence.. but i realise thats abit of effort that maybe outside the scope of what you are trying to achieve.

Mine can be public domain.

public demeanor .. thats the problem all along