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.