If you want maximally optimised PNGs, the 1st step would be to run them trough Ken Silverman's PNGOUT
http://advsys.net/ken/utils.htm
Then recompress them using the newest DEFLATE implementation: Zopfli. Use advancepng available in this package:
http://advancemame.sourceforge.net/comp-download.html choose maximal compression level to use the Zopfli algorithm and set the i parameter to 1000.
SVGs can be recompressed with advzip using the same parameters because they are zipped XML containers.