Smush.it is a Yahoo service that will help you compress the size of images in a batch before you embed them on a website or send them as email attachments. The Smush.it service can handle JPG, GIF, and PNG images that are up to 1 MB in size.
You can either upload images from the desktop or specify the image URL and smush.it will directly fetch them for optimization.
If you are uploading a GIF file, smush.it will convert them to PNG images as they are often smaller than GIF files. Thus, if you are referring to these GIF images in your CSS or HTML, you may have to change the file extensions in the code accordingly.
Internally, Yahoo’s smush.it services uses JPEGtran to remove the meta data from JPEGs and PNGCrush to shring the PNGs. For animated GIFs, it uses GIFscile to remove repeating pixels in different frames thus reducing the overall file size.
TinyPNG.org is another useful image optimization service for PNG files that reduce the file size by decreasing the number of colours in the image (24-bit PNG files are converted to 8-bit indexed colour images) but the alpha transparency is preserved.