This was recently written in Entireweb Newsletter under the heading "Ten Terrific Tips for On Page Optimisation"

7. It also helps to put your keywords within the alt tags of the images that are embedded in your HTML file. A good example would be if you have a logo that occurs at the top of the document, put the keywords in the alt text. Ie; <img src='imgs/logo.jpg' alt='Logo Harry's Pizzas - Pizza Shop Newcastle' />

In my web stats I get a lot of hits from this method
Eddie