You should apply this to png optimization. A combination of: optipng, pngout, pngcrush, DeflOpt, advpng.
It's a bit more complicated since you have to also vary the command line arguments you pass to the program. (Different color types can sometimes help, as can different filters.)
That's a pretty old tech. So old that many existing software package do that for you e.g. ImageOptim for OSX will run all of Zopfli, PNGOUT, OptiPNG, AdvPNG and PNGCrush using a nice drag-and-drop interface.
The funny thing about the software I wrote is that you can have it do that too. You just need to define what a minifier is. I do permit command line parameters.
But honestly, just use pngzopfli as post-processing and pngquant as pre-processing.
