if (document.images)
    {
      preload_image_object = new Image();
      // set image url
      image_url = new Array();
      image_url[0] = "img/button_down.png";
      image_url[1] = "img/button_up.png";
      image_url[2] = "img/sub_button_down.png";
      image_url[3] = "img/sub_button_up.png";

       var i = 0;
       for(i=0; i<=3; i++) 
         preload_image_object.src = image_url[i];
    }