Try this, it changes 2 images on mouse over, and changes them back when the mouse is moved off:
<script language="Javascript">
function change_img()
{
document.images.img1.src = "image3.gif";
document.images.img2.src = "image4.gif";
}
function change_back()
{
document.images.img1.src = "image1.gif";
document.images.img2.src = "image2.gif";
}
</script>
<a href="#" onMouseover="change_img()" onMouseout="change_back()">Mouse here to change images</a>
(You will of course need to put in the 2 images somewhere, named img1 and img2.. e.g. <img name="img1" src="image1.gif">)