if (document.images) 
{
   img1 = new Image();
   img2 = new Image();
   img3 = new Image();
   img4 = new Image();
   img1.src = "../images/topnav/credits_over.gif";
   img2.src = "../images/topnav/help_over.gif";
   img3.src = "../images/topnav/home_over.gif";
   img4.src = "../images/topnav/lessonsnav_over.gif";
  
}