code: public static String watermark(String imageUrl, int x, int y, int transparency) {
      if (imageUrl == null || imageUrl.length() == 0) {
        throw new IllegalArgumentException("Image URL must not be blank.");
      }
      if (transparency < 0 || transparency > 100) {
        throw new IllegalArgumentException("Transparency must be between 0 and 100, inclusive.");
      }
      return FILTER_WATERMARK + "(" + imageUrl + "," + x + "," + y + "," + transparency + ")";
    }
docs: This filter adds a watermark to 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.
  @param x Horizontal position that the watermark will be in. Positive numbers indicate position
  from the left and negative numbers indicate position from the right.
  @param y Vertical position that the watermark will be in. Positive numbers indicate position
  from the top and negative numbers indicate position from the bottom.
  @param transparency Watermark image transparency. Should be a number between 0 (fully opaque)
  and 100 (fully transparent).
  @throws IllegalArgumentException if {@code image} is blank or {@code transparency} is outside
  bounds.