if (document.images) {

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

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

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

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

  image6on = new Image();
  image6on.src = "images/nav/about-on.gif";

  image7on = new Image();
  image7on.src = "images/nav/contact-on.gif";

  image1off = new Image();
  image1off.src = "images/nav/home-off.gif";
  
  image2off = new Image();
  image2off.src = "images/nav/listings-off.gif";
  
  image3off = new Image();
  image3off.src = "images/nav/search-off.gif";
  
  image4off = new Image();
  image4off.src = "images/nav/questions-off.gif";
  
  image5off = new Image();
  image5off.src = "images/nav/calculator-off.gif";
  
  image6off = new Image();
  image6off.src = "images/nav/about-off.gif";
  
  image7off = new Image();
  image7off.src = "images/nav/contact-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");
  }
}