Here is a free software tool to optimize and compress or reduce file size of PNG formats without losing its original quality.
The .NET program PNGGauntlet squeezes the last bit of size out of them. Furthermore, no information is lost during this process.
It is quite simple to use. After installation, just select the files you want to optimize (or drag them onto the window), select where you want them to go, and click the button.
PNGauntlet (download it here)can also be used to convert JPG, GIF, TGA, PCX, and BMP files to PNG format.