I use the following two tools:
!AdvanceCOMP [http://advancemame.sourceforge.net/comp-readme.html]
!OptiPNG [http://optipng.sourceforge.net/]
I run the following code in each graphics directory:
advpng -z4 *.png optipng -o7 *.png
advpng -z4 *.png
optipng -o7 *.png