if (document.images) 
{
   img1 = new Image();
   img1.src = "images/tl_nav.jpg";
   
}

{
   img2 = new Image();
   img2.src = "images/tr_nav.jpg";
   
}

{
   img3 = new Image();
   img3.src = "images/bl_nav.jpg";
   
}

{
   img4 = new Image();
   img4.src = "images/br_nav.jpg";
   
}

{
   img5 = new Image();
   img5.src = "images/tl.jpg";
   
}

{
   img6 = new Image();
   img6.src = "images/tr.jpg";
   
}