var img_id_prefix = 'nav_img_';
      var nav_over = new Image();
      var nav_out = new Image();

      nav_over.src = "nav_over.gif";
      nav_out.src = "nav_out.gif";

      function toggleNavImg(img_num) {
         var image = getElement(img_id_prefix+img_num);

         if(image.src == nav_over.src)
            image.src = nav_out.src;
         else
            image.src = nav_over.src;
      }

      function getElement(id) {
         if(document.getElementById) {
            return(document.getElementById(id));
         }
         else if(document.all) {
            return(document.all[id]);
         }
         else if(document[id]) {
            return(document[id]);
         }
         else {
            return;
         }
      }