function setCaptionWidth() {
  if (document.getElementById)
    var caption = document.getElementById('caption');
  else return;
  if (caption && document.getElementsByTagName)
    var img = caption.parentNode.getElementsByTagName('img')[0];
  else return;
  
  caption.style.width = img.clientWidth + 'px';
  caption.style.visibility = 'visible';
}

addEvent(window, 'load', function () { setCaptionWidth(); });