PNG images that you include within an Xcode project are optimzed during the application build process. Good news is, this often results in a smaller application bundle. Bad news is, the resulting PNG images typically cannot be opened in Preview or other imaging software.
The tool doing the optimization is pngcrush, an open source utility available on SourceForge.
Undo pngcrush Optimizations
If you have iPhone SDK 3.2 or greater installed (and did not change the default install path), pngcrush is located here: /Developer/Platforms/iPhoneOS.platform/Developer/usr/bin/pngcrush
If you would like view or otherwise open PNG files within an Xcode build (.app file), you will need to revert the optimizations on the file, the command line syntax for this is shown below:
-revert-iphone-optimizations -q OptimizedPNG.png RevertedPNG.png
OptimizedFile.png is the name of the optimized PNG inside the .app bundle, RevertedPNG.png is the file output by pngcrush after the optimizations are removed.