This is the general problem with share alike clauses and GPL is known for adopting a very limited interpretation of their Share Alike clause. They have recognized CC0 as being compatible with GPL, which is a step. In your example, I'd say that most definitely software cannot be under CC. My perception is that the software license always prevails. Now the image in the game could be implemented with an authorization from the author (it's the best way to avoid problems in any case).