PNG (Portable Network Graphics) image optimising utility

 pngquant is a command-line conversion utility to quantize and dither truecolor
 PNG images, especially those with a full alpha channel, down to 8-bit (or
 smaller) RGBA-palette PNGs. Such images are usually two to four times smaller
 than the full 32-bit versions, and partial transparency is preserved quite
 nicely. This makes pngquant especially useful both for Web sites and for
 PlayStation 2 development, where one of the texture formats is
 RGBA-palette-based (though not PNG-compressed).
 This is the same technique used for many of the images on the Miscellaneous
 Transparent PNGs page (http://www.libpng.org/pub/png/pngs-img.html), and
 the results are often indistinguishable from the original, truecolor PNG
 images.
 .
 Optimizers (like pngcrush and optipng) optimize the compression, usually
 losslessly, while pngquant quantizes colors down to 256 (or fewer) distinct
 RGBA combinations, which is lossy.