pngcrush is an excellent batch-mode compression utility for PNG
images. Depending on the application that created the original PNGs, it can
improve the file size anywhere from a few percent to 40% or more (completely
losslessly). The utility also allows specified PNG chunks (e.g. text comments)
to be inserted or deleted, and it can fix incorrect gamma info written by
Photoshop 5.0 as well as the erroneous iCCP chunk written by Photoshop 5.5.