Wikipedia
Pngcrush
pngcrush is a free and open source command-line utility for optimizing PNG image files. It reduces the size of the file losslessly that is, the resulting "crushed" image will have the same quality as the source image.
The main purpose of pngcrush is to reduce the size of the PNG IDAT data stream by trying various combinations of compression methods and delta filters. It can also be used for various manipulations of PNG images, such as changing the bit depth, removing unwanted ancillary chunks, or adding certain chunks including gAMA, tRNS, iCCP, and textual chunks.