I found this tool recently which can be used to optimise PNG images and usually is able to reduce their file size by more than 20% (especially for files created by Photoshop etc.) It does this in a lossless way, so the image is identical. The individual savings may not be enormous but for a pakset with hundreds of PNG images it might well be significant in terms of reducing the size of the source files.
Further details here:
http://www.advsys.net/ken/util/pngout.htm
For pak128 we've been using optipng (http://optipng.sourceforge.net/), which is another tool for the same purpose. Anyway it's always nice to know new alternatives :)