function rollover() {
  if(document.getElementsByTagName) {
    var images = document.getElementsByTagName("img");
    var preImages = new Array();
      for(var i=0; i < images.length; i++) {
        if(images[i].getAttribute("src").match("_off.")) {
        var preImage = new Image();
        preImage.src =
        images[i].getAttribute("src").replace("_off.", "_on.");
        preImages.push(preImage);
        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", rollover, false);
}
else if(window.attachEvent) {
  window.attachEvent("onload", rollover);
}