document.onmouseover = handleOver;
document.onmouseout  = handleOut;

function handleOver() {
	el = window.event.srcElement;
	if (el.tagName == "IMG") { //Check if over image
		if ((el.overSrc != null) && (el.overSrc != "")) { //check if valid overSrc
			el.srcBackup = el.src;		//Back up the src
			el.src = el.overSrc;
		}
	}
}

function handleOut() {
	el = window.event.srcElement;
	if (el.tagName == "IMG") { //Check if over image
		if ((el.overSrc != null) && (el.overSrc != "") && (el.srcBackup != null)) { //Also check for valid backup
			el.src = el.srcBackup;
		}
	}
}