I make sure I credit everyone in my game, that includes you.  I talked to a person that understands more about all this legal stuff and now I understand better.

So the summary of the long story is: all sprites will be stored in my own specific internal format (it's like a zip file with light encryption), accessible via a tool I made (which will be available for download too). So my worry was if someone sold their own skins of my game (which I also don't allow), but as far as I understood, I'm not held accountable for providing the tool, I was worried I would be so, that's why I had those questions.