Mac OS X Lion v10.7.2 Xcode v4.2.1 Build 4D502 gcc version 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2336.1.00) GNU gdb 6.3.50-20050815 (Apple version gdb-1708) (Th
I had the same thing when I was adding transparent background to an image that was already part of the project.
What worked:
1) Delete app from Simulator (long press and hit X).
2) Also I had to remove old image files from project and then reload the new transparent background images into the project.
Also hit Project > Clean multiple times.