From what I gather, the CC-by-sa 3.0 share alike only pertains to the source material and transformations upon it directly. So in the case of these graphics, only they themselves or if anybody mods them, then those mods as well, must be released under the same license. Any other material you make for a game or the game itself is yours to with as that other materials' licensing dictates or as you see fit if its your own original material.

If any one else has other thoughts on this feel free to comment if I'm wrong on this.