window.onload = function(){
  var i = 0;
  var prImgSrc = "";

  //main menu
  if (document.getElementById("MainMenu")) {
    var MainRootNode = document.getElementById("MainMenu");
    var MainImgTag = MainRootNode.getElementsByTagName("img");
    for (i=0; i<MainImgTag.length; i++){
	  if (MainImgTag[i].id == "MenuOff") {
        //get preload image file name
	    prImgSrc = prImgSrc+MainImgTag[i].src.split(".jpg")[0]+"_over.jpg,";
		//mouse over event
        MainImgTag[i].onmouseover = function(){
          this.src = this.src.split(".jpg")[0]+"_over.jpg";
        }
		//mouse out event
        MainImgTag[i].onmouseout = function(){
	      if (this.src.indexOf("_over.jpg",0) != -1)
            this.src = this.src.split("_over.jpg")[0]+".jpg";
		}
	  }
	}
  }

  //image preload
  data = prImgSrc.split(",");
  prImg= new Array(data.length);
  for (i=0; i<data.length; i++){
    if (data[i]) {
      prImg[i] = new Image();
      prImg[i].src = data[i];
    }
  }

}