Yahoo introduced a web based tool called Smushit to optimize the size of web images without affecting visual quality.
Smushit allows you to upload images files or give it a URL. The tool then takes the images, optimizes them and tells you how many bytes you can save. You then get a zip of all the images for download and can replace them on your site.
Smush with different flavors:
* You can upload a bunch of pictures in your browser
* You can provide us with a list of image urls or
* You can get a Firefox Extension to optimize the images found on any web page