Hello,though the report is old the author is right. There is a easy and nice tool called 'optipng' in sid and testing that should do the job. (I'm not sure if the attached pics are up to date)
I hope someone will at least replace the images that are already png's and close the bug.
see: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=315371 greetings, Florian Ludwig