if (document.images) {
  image1on = new Image();
  image1on.src = "/images/btnspecials-upscale-on.gif";
  
  image1off = new Image();
  image1off.src = "/images/btnspecials-upscale-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");
  }
}