if (document.images)    {
  ed_on = new Image();
  ed_on.src = "images/ed_on.jpg";
  ed_off = new Image();
  ed_off.src = "images/ed_off.jpg";
  grim_on = new Image();
  grim_on.src = "images/grim_on.jpg";
  grim_off = new Image();
  grim_off.src = "images/grim_off.jpg";
  fosters_on = new Image();
  fosters_on.src = "images/fosters_on.jpg";
  fosters_off = new Image();
  fosters_off.src = "images/fosters_off.jpg";
  }

function imgOn(imgName) {
       if (document.images) {
          document[imgName].src = eval(imgName + "_on.src");
		}
}
function imgOff(imgName) {
       if (document.images) {
          document[imgName].src = eval(imgName  + "_off.src");
       }
}