Install via homebrew:
brew install pngquant oxipng
Reduce image to a maximum of 50 colors:
pngquant --quality 20-50 image.png
Then apply oxipng:
oxipng -o 3 -i 0 --strip safe *-fs8.png
The -o 3 defines level of optimization (4 is the highest recommended).
-i 0 causes it to remove interlacing - “Interlacing can add 25-50% to the size of an optimized image” according to the README.
--strip safe strips out any image metadata that is guaranteed not to affect how the image is rendered.