If you want to compress the PNG files for wiiflow, then you can use the batch file included and the following tools you can download by yourself.

optipng:
http://optipng.sourceforge.net/
pngout:
http://advsys.net/ken/utils.htm
advpng:
http://advancemame.sourceforge.net/comp-download.html
DeflOpt:
http://www.walbeehm.com/download/