code: public static String frame(String imageUrl) {
      if (imageUrl == null || imageUrl.length() == 0) {
        throw new IllegalArgumentException("Image URL must not be blank.");
      }
      return FILTER_FRAME + "(" + imageUrl + ")";
    }
docs: This filter uses a 9-patch to overlay the image.
  
  @param imageUrl Watermark image URL. It is very important to understand that the same image
  loader that Thumbor uses will be used here.