// preload back images
if (document.images)
{
  pic0= new Image(999,80); 
  pic0.src="images/top.png";
  
  pic1= new Image(116,482); 
  pic1.src="images/left.png"; 

  pic2= new Image(116,482); 
  pic2.src="images/right.png"; 
  
  pic3= new Image(999,134); 
  pic3.src="images/bottom.png";
  
 }