Download PNGGauntlet, to optimize PNG graphics
PNGGauntlet will allow you to optimize (reduce the size of) PNG files.
It squeezes the last bit of size out of them. Most important is that no information is lost during this process.
Select the files you want to optimize (or drag and drop them to the program’s window), select where you want them to go (or overwrite the source), and click the “Optimize” button.
PNGauntlet can also be used to convert JPG, GIF, TGA, PCX, and BMP files to PNG.