mozjpeg goal is to reduce the size of JPEG files without reducing quality or compatibility. As of today there is no GUI for mozjpeg but you can easily run it from the command line and compress several files at once.

Installing mozjpeg under OSX

The easiest way to get mozjpeg is through Homebrew. By installing Homebrew and then running brew install mozjpeg you will get the latest version of mozjpeg compiled on your machine.

Once compiled, you can start using it by running the cjpeg executable inside the mozjpeg installation directory. It will not be available globally because it could interfere with other JPEG compressors.

Compress all images inside a directory

Imagine you have this folder structure
assets/
  - images/
  - optimized/

Open the Terminal, cd into the images folder and then run the following command:

find ./ -name "*.jpg" -exec /usr/local/Cellar/mozjpeg/2.1/bin/cjpeg -quality 80 -outfile "../optimized/{}" {} \;

Obviously, cjpeg path should reflect your mozjpeg installation directory. After few seconds (or minutes, depending on the number of images) your compressed images will be inside the optimized folder.