How does PunyPNG work?
PunyPNG uses several web optimization techniques to reduce the file sizes of your images. It first analyzes the content of your image and figures out the best image format to use.
Depending on the file, PunyPNG will automatically convert your GIF into a PNG format and then further compress that. For JPEGs, sometimes it's better to make it a PNG, and other times it's better to keep it as JPEG and use jpeg-tran to optimize it. PunyPNG makes all those decisions for you and ensures the best possible results.
Other tools have picky requirements about which file formats they support. With PunyPNG, just fire and forget. It supports the most popular formats and is constantly improving its arsenal of optimization techniques such as dirty transparency and animated PNG support (coming soon).