Optimisation could be much better

[gradha]

Reading through the readme I was shocked at the minor optimisation gains. If you pass the 500kb image through https://pngquant.org you get a 197kb png. Maybe pngquant (or other tools) would be more appropriate than the current choice of pngtastic+zopfli.

[peterdk]

pngquant uses lossy compression, so it's more like JPG. It doesn't result in identical image.