Well the issue is GPLv3 is not compatible with GPLv2 directly. Usually the images fall under the same license and the code. Somewhat. The easy way to look at is: BSD/MIT licenses say Do anything you want with my work so long as you credit me (no restrictions on your license, doesn't even have to be open source) GPL/Apache say Do what you want to as long as you release all the derivative works under the same license.