Have a look at a good mouseover tutorial from JH's http://www.vortex-webdesign.com/help/
A Rollover Builder is at http://www.webreference.com/js/tools/rollovers/
Isofarro says --
There are a number of good ways of doing mouse rollovers.
Avoid any mouse rollovers that use eval(), since they are notoriously slow.
<a href="myTeddyBear.html" onMouseOver="document.myTeddy.src='smileyBear.gif'" onMouseOut="document.myTeddy.src='unhappyBear.gif'"> <img name="myTeddy" src="unhappyBear.gif" alt="My Teddy Bear"> </a>