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; 
        } 
    } 
}