Putt scriptet mellom HEAD-taggene:

<script type="text/javascript">
function SimpleSwap(el,which) {
el.src=el.getAttribute(which || "origsrc");
}

function SimpleSwapSetup() {
var x = document.getElementsByTagName("img");
for (var i=0;i<x.length;i++){
var oversrc = x[i].getAttribute("oversrc");
if (!oversrc) continue;

// preload image -
// comment the next two lines to disable image pre-loading
x[i].oversrc_img = new Image();
x[i].oversrc_img.src=oversrc;

// set event handlers
x[i].onmouseover = new Function("SimpleSwap(this,'oversrc');");
x[i].onmouseout = new Function("SimpleSwap(this);");

// save original src
x[i].setAttribute("origsrc",x[i].src);
}
}

var PreSimpleSwapOnload =(window.onload)? window.onload : function(){};
window.onload = function(){PreSimpleSwapOnload(); SimpleSwapSetup();}
</script>

Bildelinken setter du der du vil ha den:
<img src="bilder/bok11.gif" oversrc="bilder/buffy.jpg">