This is a neat little trick that uses CSS and JavaScript to pull of an image swap effect. The script is pretty easy to understand and can be adapted to your needs even by a fairly inexperienced user. An easy way to find your way around is to open the page in a browser, right click and do view source then file print from the opened note pad. This will give you a hard copy so you can see how it all works.

These pages will take upto 60 seconds to load with 52k dial up.