if (document.images)
    {
      preload_image_object = new Image();
      // set image url
      image_url = new Array();
      image_url[0] = "/images/slide1.jpg";
      image_url[1] = "/images/slide2.jpg";
      image_url[2] = "/images/slide3.jpg";
      image_url[3] = "/images/slide4.jpg";

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