I use the following two tools:
I run the following code in each graphics directory:
advpng -z4 *.png optipng -o7 *.png
See horde/framework/devtools/horde-compress-pngs.php http://cvs.horde.org/framework/devtools/horde-compress-pngs.php.
horde/framework/devtools/horde-compress-pngs.php