Would be excellent is as well as the deault zip compression you also supported ZOPFLI for PNG images. For example https://twitter.com/ericlaw/status/521295279232794624 " ZopFli recompression of PNGs is yielding ~5%-13% improvement in quick tests.".
You already have the ZOPFLI code as it's part of the WebP format.
Add ZOPFLI compression as an option for PNG files
Moderators: helmut, XnTriq, xnview