function smartRollover() {
    if(document.getElementsByTagName) {
        var images = document.getElementsByTagName("img");

        for(var i=0; i < images.length; i++) {
            if(images[i].getAttribute("src").match("_off.")) {
                fileName = new Array(images[i].getAttribute("src").replace("_off.", "_on."));
                preImages = new Array();
                for (j=0; j<fileName.length; j++) {
                    preImages[j] = new Image();
                    preImages[j].src = fileName[j];
                }
                images[i].onmouseover = function() {
                    this.setAttribute("src", this.getAttribute("src").replace("_off.", "_on."));
                }
                images[i].onmouseout = function() {
                    this.setAttribute("src", this.getAttribute("src").replace("_on.", "_off."));
                }
            }
        }
    }
}

if(window.addEventListener) {
    window.addEventListener("load", smartRollover, false);
}
else if(window.attachEvent) {
    window.attachEvent("onload", smartRollover);
}