Normal2 = new Image();
Normal2.src = "images/2_off.jpg";   
Highlight2 = new Image();
Highlight2.src = "images/2_on.jpg";

Normal3 = new Image();
Normal3.src = "images/3_off.jpg";   
Highlight3 = new Image();
Highlight3.src = "images/3_on.jpg";

Normal4 = new Image();
Normal4.src = "images/4_off.jpg";   
Highlight4 = new Image();
Highlight4.src = "images/4_on.jpg";

Normal5 = new Image();
Normal5.src = "images/5_off.jpg";   
Highlight5 = new Image();
Highlight5.src = "images/5_on.jpg";

Normal6 = new Image();
Normal6.src = "images/6_off.jpg";   
Highlight6 = new Image();
Highlight6.src = "images/6_on.jpg";

function changeImg (imgId, imgObject) {
  window.document.images[imgId].src = imgObject.src;
}