MozJPEG is a project from Mozilla aimed to improve JPEG compression while keeping full backwards compatibility with current decoders. Recently they released version 3 of the library. Previous version reduced files by 5% on average compared to jpeg-turbo, the JPG compression library it is based on. The new version contains more improvements, such as reducing compression artifacts for text on white background, and better quality for high-resolution images.

Installing and using MozJPEG in OS X

MozJPEG can be installed on a Mac via Homebrew package manager by typing: 
brew install mozjpeg
Since mozjpeg is a replacement for libjpeg, to avoid any compatibility issues with programs using libjpeg or its utilities, Homebrew won't create symlinks to replace libjpeg with it. For convenience, I created the following links to cjpeg and jpegtran binaries from mozjpeg: 
ln -s /usr/local/Cellar/mozjpeg/3.0/bin/cjpeg /usr/local/bin/mozcjpeg

ln -s /usr/local/Cellar/mozjpeg/3.0/bin/jpegtran /usr/local/bin/mozjpegtran
(Note: replace 3.0 with whatever version you have installed, e.g. 3.1) This way calling cjpeg will execute the libjpeg's utility, while calling mozcjpeg will use the Mozilla's version. All following examples will use binaries linked like this.

Converting PNG to JPEG with MozJPEG

To convert PNG to JPG, we'll first use convert utility from ImageMagick (you probably have it installed; if not, run brew install imagemagick) to convert PNG to a format accepted by cjpeg, and then pipe the result of conversion into MozJPEG's version of cjpeg to get the JPEG file: 
convert filename.png pnm:- | mozcjpeg -quality 90 > filename.jpg
Tweak quality parameter (recommended range is 50 to 90) until you get the results you need.

Optimizing and re-compressing JPEGs

If you want to make your JPEGs smaller with MozJPEG optimizer, you can use jpegtran utility from its distribution: 
mozjpegtran -outfile output.jpg -optimise -copy none input.jpg
(again, we're using mozjpegtran which is symlinked to jpegtran binary from mozjpeg, see above.) JPEGtran uses lossless optimizations, so there will be no change in quality. If you'd like to re-compress JPEGs, losing some quality, but making it even smaller, you can again use convert utility with cjpeg: 
convert filename.jpg pnm:- | mozcjpeg -quality 70 > filename-optimized.jpg
Again, tweak quality option for your taste.

Reading

Project information

GitHub: https://github.com/mozilla/mozjpeg Authors: Mozilla and libjpeg-turbo contributors License: 3-clause BSD