if (document.images) {

  image1on = new Image();
  image1on.src = "images/nav/btn_residential_on.gif";
  
  image2on = new Image();
  image2on.src = "images/nav/btn_ranches_on.gif";

  image3on = new Image();
  image3on.src = "images/nav/btn_commercial_on.gif";

  image4on = new Image();
  image4on.src = "images/nav/btn_bldg_lots_on.gif";

  image1off = new Image();
  image1off.src = "images/nav/btn_residential_off.gif";
  
  image2off = new Image();
  image2off.src = "images/nav/btn_ranches_off.gif";
  
  image3off = new Image();
  image3off.src = "images/nav/btn_commercial_off.gif";
  
  image4off = new Image();
  image4off.src = "images/nav/btn_bldg_lots_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");
  }
}