code: public boolean photos_addTag(Long photoId, Integer taggedUserId, Double xPct, Double yPct)
      throws FacebookException, IOException {
      return photos_addTag(photoId, xPct, yPct, taggedUserId, null);
    }
docs: Adds a tag to a photo.
  @param photoId The photo id of the photo to be tagged.
  @param xPct The horizontal position of the tag, as a percentage from 0 to 100, from the left of the photo.
  @param yPct The vertical position of the tag, as a percentage from 0 to 100, from the top of the photo.
  @param taggedUserId The list of photos from which to extract photo tags.
  @return whether the tag was successfully added.