pre-commit

#!/bin/sh
compression=80
hasExif=`which exiftool`

pngs=`git status -s | grep .png | awk '{print $2}'`
arr=($pngs)
if [ -n "$arr" ]; then
    echo "Compressing png images by $compression%..."

    for png in "${arr[@]}"
    do
        oldSize=`exiftool -FileSize $png | awk '{print $4$5}'`
        pngquant --quality=80 --ext=.png --force $png
        newSize=`exiftool -FileSize $png | awk '{print $4$5}'`
        git add $png
        echo "    Compressed $png from $oldSize to $newSize"
    done
fi

exit 0

Git pre commit hook to compress (modified) png files, and automatically add them to the commit. This assumes you are exporting the data from a PSD / image editing program every time you make a change, so you will not re-compress a compressed image.

put the gist in .git/hooks/pre-commit

brew install pngquant
brew install exiftool
chmod +x .git/hooks/pre-commit

Remember, you cannot commit git hooks, so you must add this to any machine you wish to have it on (yes, git is dumb)