Like many I want to make an open source game, and like many I am a bit overwhelmed by the licenses.  
What I want to do is make an open source 2D Android game. I have made many demo project but have always had problems with my programmer art. Friends can help with interface and layout, but the sprites have always been a problem for me. Upon seeing this site I was very excited. I have never made an open source project before, and wanted to start with a simple Android game. It seems if I use an open license such as the GPL I can put the game up on github. But most projects I see do not have their art assets there. Is this common place? Also some people I talked to said github wasn’t that open.

So much question boils down to this. If I made an Android game used the GPL 3 License and used compatable art, and put it all(including the art) with the license in github, would this not violate anything? Or does the license mean that if they download my project the code must go with the executable(like Xonotic). I am concerned with how to properly attribute the art and open credit because I would hate to wrong an artist who produced something only to make it look like I plagiarized it.

Lastly is it possible to semi manipulate a given art piece? Such as make the background transparent. Or does it have to be the derived works Creative Commons license to do that?

Thank you for reading this long post