Pngcrush
Pngcrush is a free command line tool to optimize the compression of PNG image files, which in most cases reduces the file size. The new compression takes place without any loss of quality .
Pngcrush only recompresses the image data and retains the color format . With many images it is also possible to choose a more compact color format without loss (e.g. 4-bit gray levels instead of a color table with 16 gray tones). The program pngrewrite was created for this, which should be executed before pngcrush.
The tool is developed by Glenn Randers-Pehrson and is available for Linux , macOS and Windows , among others . The current source code version 1.8.13 was released on August 29, 2017.
There are other programs for recompressing PNG files with mostly slightly more compact results, such as OptiPNG or PNGOUT .