He's using some CSS which would download the images, but not display them. In the section where it says, images go here, you would put <img> tags for each of the images. The browser would download them as part of loading the page, and when you go to rollover the images, they would already be downloaded, and display instantly. This solution wouldn't require any changes to your rollover script.
If you want to do it completely in javascript, have a look at some of the scripts over at hotscripts.com:
http://www.hotscripts.com/JavaScript...ges/index.html