if (document.images)
    {
      preload_image_object = new Image();
      // set image url
      image_url = new Array();
      image_url[0] = "images/bg1.jpg";
      image_url[1] = "images/bg2.jpg";
      image_url[2] = "images/bg3.jpg";

       var i = 0;
       for(i=0; i<=2; i++) 
         preload_image_object.src = image_url[i];
    }