i wrote this. it uses an image object in javascript to store the image you moused over then swaps it back again. this requires no name or id attributes and only one javascript image per button. it's really quick to use.
it works on simple websites without flicker. some navigation systems are too complex for this. put the javascript in the head of your doc. this won't work on older browsers but wont throw up any errors because the events are ignored in the img tag.
<script type="text/javascript">
var iHolder = new Image();
var iButtonA = new Image(); iButtonA.src = "something.gif";
var iButtonB = new Image(); iButtonB.src = "somethingelse.gif";
</script>