code: public T photos_addTags(Long photoId, Collection<PhotoTag> tags)
      throws FacebookException, IOException {
      assert (photoId > 0);
      assert (null != tags && !tags.isEmpty());
  
      JSONArray jsonTags = new JSONArray();
      for (PhotoTag tag : tags) {
        jsonTags.add(tag.jsonify());
      }
  
      return this.callMethod(FacebookMethod.PHOTOS_ADD_TAG,
                             new Pair<String, CharSequence>("pid", photoId.toString()),
                             new Pair<String, CharSequence>("tags", jsonTags.toString()));
    }
docs: Adds several tags to a photo.
  @param photoId The photo id of the photo to be tagged.
  @param tags A list of PhotoTags.
  @return a list of booleans indicating whether the tag was successfully added.