var mainRollover = document.getElementById("mainRollover");


function getElementsByStyleClass(className) {
  var all = document.all ? document.all :
    document.getElementsByTagName('*');
  var elements = new Array();
  for (var e = 0; e < all.length; e++)
    if (all[e].className == className)
    elements[elements.length] = all[e];
  return elements;
}


var allLinks = getElementsByStyleClass("rollover");
var imageObj = new Image(); //tempObj for Preload
for (var i = 0; i < allLinks.length; i++) {
  allLinks[i].onmouseover = linkMouseOver;
  imageObj.src = allLinks[i].href;
}

function linkMouseOver() {
  mainRollover.src = this.href;
}