if (document.images) {
  image1on = new Image();
  image1on.src = "Images/button-about-us-on.jpg";
 
  image2on = new Image();
  image2on.src = "Images/button-services-on.jpg";
 
  image3on = new Image();
  image3on.src = "Images/button-estimates-on.jpg";
 
  image4on = new Image();
  image4on.src = "Images/button-contacts-on.jpg";
 
  image1off = new Image();
  image1off.src = "Images/button-about-us-off.jpg";
 
  image2off = new Image();
  image2off.src = "Images/button-services-off.jpg";
 
  image3off = new Image();
  image3off.src = "Images/button-estimates-off.jpg";
 
  image4off = new Image();
  image4off.src = "Images/button-contacts-off.jpg";
 
}
 
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");
  }
}