function rollover() {
  obj = getElementsByClassName('rollover');
  for (var i in obj) {
    obj[i].imgsrc = obj[i].src;
    obj[i].imgext = obj[i].imgsrc.substring(obj[i].imgsrc.lastIndexOf('.'), obj[i].imgsrc.length);
    obj[i].imgsrc_o = obj[i].imgsrc.replace(obj[i].imgext, '_o'+obj[i].imgext);
    obj[i].onmouseover = function () { this.src = this.imgsrc_o; }
    obj[i].onmouseout = function () { this.src = this.imgsrc; }
    imgs[imgs.length] = obj[i].imgsrc_o;
  }
}
onload_items[onload_items.length] = rollover;

function preloadimages(){
  var onImgs = new Array();
  for (field in imgs) {
    onImgs[field] = new Image();
    onImgs[field].src = imgs[field];
  }
}
onload_items[onload_items.length] = preloadimages;