Woooooo!
I fixed it. I changed the bitmap size from 1024 x 1024 down to 256 x 256. That seemed to resolve the problem, so it must have been somehow memory related. Strange that it worked the first time around, bit only failed after exiting and reloading the game. 256 is actually big enough to capture all the detail I need, so 1024 was kind of a waste of memory anyway.