function roll(img_name, img_src)
   {
   document[img_name].src = img_src;
   }

image1 = new Image();
image1.src = "images/staff_on.png";

image2 = new Image();
image2.src = "images/locations_on.png";

image3 = new Image();
image3.src = "images/services_on.png";