if (document.images)
    {
      preload_image_object = new Image();
      // set image url
      image_url = new Array();
      image_url[0] = "photos/bridge.jpg";
      image_url[1] = "photos/crane.jpg";
      image_url[2] = "photos/turbine.jpg";
      image_url[3] = "photos/bridge2.jpg";
      image_url[4] = "photos/turbines2.jpg";
      image_url[5] = "photos/solarpanels.jpg";
      image_url[6] = "photos/windturbines.jpg";

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