function doLinkEvents() {
  
  var imgs = getElementsByClassName("rollover");
  
  for(var i = 0; i < imgs.length; i++) {
    
    var imgFormat = getImgFmt(imgs[i]);
    
    var onImg = imgs[i].src.substr(0,imgs[i].src.lastIndexOf(imgFormat)) + "_rollover" + imgFormat;
    
    new Image().src = onImg;
    
    imgs[i].rollover = onImg;
    
    imgs[i].rollout = imgs[i].src;
    
    imgs[i].onmouseover = function() {
      this.src = this.rollover;
    }
    
    imgs[i].onmouseout = function() {
      this.src = this.rollout;
    }
  }
  
}

function getImgFmt(img) {
  return img.src.match(/(\.gif$)|(\.jpg$)|(\.png$)/ig);
}

addLoadEvent(doLinkEvents);