//this function loads an alternative picture if the
//original picture is not available.
//NOTE: the two pictures must have the same size, otherwise the alternative picture will be
//      resized to the values defined by the height= and width= attributes

function onImgError(source,picture)
{
  var picture;

  //alternative path+picture
  source.src = picture;

  // disable onerror to prevent endless loop
  source.onerror = "";
  return true;
}