ImageOptim by Kornel Lesiński and contributors is a GUI for 3rd party utilities:

- OptiPNG by Cosmin Truta,
- PNGCrush by Glenn Randers-Pehrson,
- Zopfli by Jyrki Alakuijala, Lode Vandevenne,
- AdvPNG by Andrea Mazzoleni, Filipe Estima,
- Jpegoptim by Timo Kokkonen,
- Gifsicle by Eddie Kohler,
- and PNGOUT by Ken Silverman.

ImageOptim can be redistributed and modified under GNU General Public License version 2 or later. Bundled PNGOUT is not covered by the GPL and is included with permission of Ardfry Imaging, LLC.