function rollOver(elm, overImage) {
  overImage = overImage || "_o";
  this.img = elm;
  this._image = elm.src;
  var src = elm.src;
  this.image = src.replace(/(.*)(\.[jpg|jpeg|gif|png])/g, "$1" + overImage + "$2");
}

rollOver.prototype.over = function() {
  var self = this;
  addEvent(this.img, "mouseover", function(e) {
    var el = e.target || e.srcElement;
    el.src = self.image;
  });
}

rollOver.prototype.out = function() {
  var self = this;
  addEvent(this.img, "mouseout", function(e) {
    var el = e.target || e.srcElement;
    el.src = self._image;
  });
}

addEvent(window, "load", function() {
  var menu = document.getElementById("menu");
  var ul = menu.getElementsByTagName("ul")[0];
  var lis = ul.getElementsByTagName("li");
  for (var i = 0; i < lis.length; i++) {
    var img = lis[i].getElementsByTagName("img")[0];
    if (img) {
      var ro = new rollOver(img);
      ro.over();
      ro.out();
    }
  }
});