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."))
        {
          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);
}