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).
I'm in complete agreement with you, but I think we wouldn't need to rely on personal interpretations and recommended practices if the SA licenses stated clearly something akin to:
If a copy or a derivative work of the licensed material is to be included in a piece of software under the General Public License or other strong copyleft license, then it must be licensed under that license for compliance.
I don't speak legalese, so I'm sure that text is full of holes and possible misinterpretations, but surely a group of experts could come up with something legally viable in that spirit.
