The PNG images found on the iPhone cannot be viewed by a standard image viewer. Apple, for some unknown reason, implemented a non-standard PNG format that includes an extra data before the PNG header and compressed image data without the traditional headers and footers. In addition, the red blue pixels are flipped (BGRA instead of RGBA). All iPhone PNG images appear to follow this format.
http://modmyi.com/wiki/index.php/Iphone_PNG_images
http://modmyi.com/wiki/index.php/Iphone_PNG_images