if (document.images) {

  image1on = new Image();
  image1on.src = "images/nav/home-on.gif";
  
  image2on = new Image();
  image2on.src = "images/nav/architectural-on.gif";

  image3on = new Image();
  image3on.src = "images/nav/scenic-on.gif";

  image4on = new Image();
  image4on.src = "images/nav/colorado-on.gif";

  image5on = new Image();
  image5on.src = "images/nav/argentina-on.gif";

  image1off = new Image();
  image1off.src = "images/nav/home-off.gif";
  
  image2off = new Image();
  image2off.src = "images/nav/architectural-off.gif";
  
  image3off = new Image();
  image3off.src = "images/nav/scenic-off.gif";
   
  image4off = new Image();
  image4off.src = "images/nav/colorado-off.gif";
  
  image5off = new Image();
  image5off.src = "images/nav/argentina-off.gif";
  
}
function turnOn(imageName) {
  if (document.images) {
    document[imageName].src = eval(imageName + "on.src");
  }
}

function turnOff(imageName) {
  if (document.images) {
    document[imageName].src = eval(imageName + "off.src");
  }
}