pngquant - Command-line utility and a library for lossy compression of PNG images

pngquant is a PNG compressor that significantly reduces file sizes by converting images to a more efficient 8-bit PNG format with alpha channel (often 60-80% smaller than 24/32-bit PNG files). Compressed images are fully standards-compliant and are supported by all web browsers and operating systems.

It supports high-quality palette generation using a combination of vector quantization algorithms. It uses unique adaptive dithering algorithm that adds less noise to images than the standard Floyd-Steinberg.