Skipping the 'CompressResources' build step for Xcode iPhone apps
The iphoneos-optimize script converts PNG files into a nonstandard format that is optimized for display on the iPhone. The script will convert any files with the png extension that it finds inside your app bundle.
I had a similar problem, and solved it by giving my file the extension _png (i.e., prefixed an underscore). iphoneos-optimize ignored it and left it a regular PNG file. If you can deal with it that way, it's probably a lot safer than mucking about with the build scripts.