if (document.images) {

  img1on=new Image();
  img1on.src="../images/arrow-left-on.gif";
  img2on=new Image();
  img2on.src="../images/arrow-right-on.gif";

  img1off=new Image();
  img1off.src="../images/arrow-left-off.gif";
  img2off=new Image();
  img2off.src="../images/arrow-right-off.gif";
}

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");
      }
}