btn1 = new Image();
btn1.src = "html/images/btn1.jpg";     
btnhov1 = new Image();
btnhov1.src = "html/images/btn1h.jpg"; 

btn2 = new Image();
btn2.src = "html/images/btn2.jpg";    
btnhov2 = new Image();
btnhov2.src = "html/images/btn2h.jpg"; 

btn3 = new Image();
btn3.src = "html/images/btn3.jpg";    
btnhov3 = new Image();
btnhov3.src = "html/images/btn3h.jpg"; 

btn4 = new Image();
btn4.src = "html/images/btn4.jpg";     
btnhov4 = new Image();
btnhov4.src = "html/images/btn4h.jpg"; 

btn5 = new Image();
btn5.src = "html/images/btn5.jpg";     
btnhov5 = new Image();
btnhov5.src = "html/images/btn5h.jpg"; 

btn6 = new Image();
btn6.src = "html/images/btn6.jpg";     
btnhov6 = new Image();
btnhov6.src = "html/images/btn6h.jpg"; 

function toggle(nr, pic) {
  window.document.images[nr].src = pic.src;
}