wozu dw für so ein simplen spass.
<script>
function dom(pic,id){
document.getElementById(id).setAttribute('src',pic);
}
</script>
<img src="pic.gif" id="p1" />
<a href="#" onMouseOver="dom('neu.gif','p1')" onMouseOut="dom('pic.gif','p1')">wechsel</a>
auch für alte browser
<script>
over = new Image();
over.src = 'neu.gif';
old = new Image();
old.src = 'pic.gif';
function dom(pic,id){
document.images[id].src = pic.src;
}
</script>
<img src="pic.gif" id="p1" />
<a href="#" onMouseOver="old('neu.gif',over)" onMouseOut="old('pic.gif',old)">wechsel</a>