SmushIt removes all extraneous data from image files without reducing image quality. Non-lossy image optimization tools that are image format specific are used to minimize image file size and maximize image quality. It works with JPGs, PNGs, GIFs, and animated GIF files. When the image or images are done processing, which is super fast, they are presented for downloaded in a zip archive.
Read, Write, Web tested SmushIt on some websites and found that it reduced image size 10 to 45%. SmushIt is a simple, fast tool for image optimization that will come in handy for bloggers, developers, and publishers alike.
Stoyan Stefanov, one of the two SmushIt developers, posted on his blog about exactly how SmushIt works. Nicole Sullivan also explains more about SmushIt.