Firstly, I guess this is a clear example of why the GPL doesn't work well for content rather than code (just as the CC licences don't work well for code rather than content).
Secondly, the question of how a GPLv2 image combines with the CC-BY-SA content comes down to whether the larger work that includes that image is a derivative work of that image. If it is, then as Kevin says above, the licences are incompatible, and the resulting work cannot be distributed.
Consider the judgement in Drauglis v. Kappa Map Group, LLC. Although the writeup linked is rather emotive and misspells the plaintiff's name throughout, it has the summary correct: the use of a copylefted image as the main cover photo for a book made neither the entire book nor the cover alone a derivative work of the cover photo. As the judgement put it, "the Court finds that neither the atlas nor its cover constitutes a derivative work subject to the ShareAlike requirement".
My personal feeling is therefore that you can have GPLv2 images displayed in your game alongside CC-BY-SA images (and sound) as long as you don't deliberately try to create derivative images or other works that combine GPLv2 content and CC-BY-SA content. You will, of course, need to make sure that the GPLv2 content (and, for that matter, the CC-BY-SA content) can be obtained in isolation by anyone who wishes to avail themselves of the benefits of the licence.
But IANAL/IANYL, so you should seek a qualified legal opinion before you put yourself in harm's way.
