function swap(id, newsrc){
  var theImage = locateImage(id);
  if (theImage) {
    theImage.src = newsrc;
  }
}
  
function locateImage(name) {
  var theImage = false;
  if (document.images){
    theImage = document.images[name];
  }
  if (theImage) {
    return theImage;
  }
  return (false);
}