In mid November 2007 I optimized the images on my website. The site received 12177 visitors and used 5.78GB of bandwidth. However, December was the first full month of the optimized images. The site received 14892 visitors yet used only 4.54GB of bandwidth. That equates to a 22% increase in visitors yet a 21% decrease in bandwidth. Of course, my site is a photoblog so this makes the difference more dramatic
And be sure to take full advantage of the SEO opportunities offered by your images: • Write some concise ALT text, with appropriate keywords, for the image. • If appropriate, link the image to another desirable, relevant page. For example, on my home page I link my photo to the Biographical Data page. Clicking on the photo image sends you to the Bio page.
i do it sometimes when im running out of bandwidth on my image host, i can reduce my 100-180gb day usage down by about 20-30% with only small degrade of image quality.