function rollover() {
	if (!document.getElementById) return	
	var img_org;	
	var img_a = new Array();	
	var img_events = document.getElementsByTagName('img');
	
	for (var i = 0; i < img_events.length; i++) {
	
		if (img_events[i].getAttribute('mo_src')) {	
			img_a[i] = new Image();	
			img_a[i].src = img_events[i].getAttribute('mo_src');
	
			img_events[i].onmouseover = function() {	
				img_org = this.getAttribute('src');	
				this.setAttribute('src',this.getAttribute('mo_src'));	
			}
	
			img_events[i].onmouseout = function() {	
				this.setAttribute('src',img_org);	
			}
		}
	}
}

onload=rollover;

// EXAMPLE
// <img src="before.gif" mo_src="after.gif"><br>
// <a href="#"><img src="on.gif" mo_src="myimages/off.gif"></a>