tl;dr: Use pngquant to minify PNGs.

brew install pngquant

wget https://upload.wikimedia.org/wikipedia/commons/5/53/Wikimedia-logo.png

pngquant Wikimedia-logo.png -o Wikimedia-logo.min.png

23K Wikimedia-logo.min.png
31K Wikimedia-logo.png

The pngquant tool was able to optimize the file by (31 - 23) / 31 * 100 = 26%; this is a material improvement.