Pngcrush

Pngcrush is a utility that can be used to squeeze a few more bytes out of an ".png" file. See the sourceforge page on pngcrush for details.

"pngcrush -cc" will reduce an image to the actual number of colors in the image, if a grayscale picture is stored as a truecolor image with alpha, or if a black & white image is stored as grayscale.

See also optipng, a newer utility offering roughly the same functionality. The general subject of PNG optimisation and tools is discussed more in depth in this article: A guide to PNG optimization.