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

function handleOver() { 
el = window.event.srcElement; 
if (el.tagName == "IMG") {  
if ((el.overSrc != null) && (el.overSrc != "")) { 
el.srcBackup = el.src; 
el.src = el.overSrc; 
}
}
}

function handleOut() { 
el = window.event.srcElement;
if (el.tagName == "IMG") { 
if ((el.overSrc != null) && (el.overSrc != "") && (el.srcBackup != null)) {
el.src = el.srcBackup;
}
}
}