if (document.images) {
  button_off= new Image(6,6);
  button_off.src="img/buttonblue.png";
  
  button_on= new Image(6,6);
  button_on.src="img/buttongreen.png";
}

function change(picName,imgName) {
   if (document.images) {
      imgOn=eval(imgName + ".src");
      document[picName].src= imgOn;
    }
 }