PNG image files are often not compressed with the optimal compression setting, however a one line command can fix this (and save some space).
Basically, one just needs to have MacPorts and the optipng port installed, and run the following command:
mdfind -0 -onlyin ~ "kMDItemContentType=public.png" | xargs -n 5 -0 optipng -preserve
Mac OS X Hints
http://hints.macworld.com/article.php?story=20111122135810992