This is a handy tool provided for us from Spinwave. We use it here all the time. (You may wish to bookmark this page.)
Some of our page loading times have been cut in half simply by optimizing our jpegs with the Jpeg Optimizer. In fact, the background jpeg we use on the auniverse home page (and on many other pages of our site here) was optimized using the following tool. We reduced it from a whopping 35 kb size down to a 13.4 kb size, drastically increasing our page load speeds!
Don't forget .gif is also one of the most popular format of graphics you find on the Internet. Click here to optimize your gifs free here now.