There is a command-line utility that is run during Xcode app builds called 'pngcrush'. It was originally written by somebody else, then modified a bit for Apple's use. It tries to further compress png files and, I think, reorganizes them so that they can be more efficiently rendered by the iPhone's GPU. Like, maybe so they can be drawn bottom-to-top, rather than the usual top-to-bottom? I read about this once, but it was years ago, and I can't remember all the details.