$(function(){
  var imgs = $(".rollover").get();
  for(var i = 0; i < imgs.length; i++) {
    if(imgs[i].src.match("_rollover")) {
      // do nothing
    } else {
      var imgFormat = imgs[i].src.match(/(\.gif$)|(\.jpg$)|(\.png$)/ig);
      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;
      }
    }
  }
});