In the continuing quest for standards compliance and clean code, a "rollover" that doesn't require client side JavaScript is not only cool as cats pajamas, but it does somethings a lot better too.
For instance, Dreamweaver has been known to miss the "OnMouseOver" images when moving files and adjusting links. (Forget about using wget, it does not interpret JavaScript so it never gets all the images.)
This idea is lifted straight from the master, Eric Meyer. He has a complete explanation of the technique at the O'Reilly Network.
Internet Explorer (version 6 point whatever is current today) kind of flashes the images when changing them a bit, but the effect is not too bad. Opera 7 handles the hover event smoothly. You can see my sketch of this idea at http://www.chathamselfstorage.com/rates.html