gnome-fixpng-thumbnailer thumbnails a very modified version of PNG files used by Apple in their iOS build system. More information about the format is available from: http://coding.scribd.com/2011/09/07/shrink-your-ios-app-by-turning-png-compression-up-to-11/ and: http://iphonedevelopment.blogspot.co.uk/2008/10/iphone-optimized-pngs.html The code is based on "iphone-fixpng.tgz", from unknown writers, but licensed under the GPL v2.