if (document.images) 
{
   img1 = new Image();
   img2 = new Image();
   img3 = new Image();
   img4 = new Image();
   img1.src = "1.jpg";
   img2.src = "images/phase5.jpg";
   img3.src = "images/phase6.jpg";
   img4.src = "images/phase7.jpg"; 
}