Adds a watermark to an image

def watermark(img, mark, position=(0, 0), opacity=1, scale=1.0, tile=False,
              greyscale=False, rotation=0, return_name=False, **kwargs):
    """Adds a watermark to an image"""

    if opacity < 1:
        mark = reduce_opacity(mark, opacity)

    if not isinstance(scale, tuple):
        scale = determine_scale(scale, img, mark)

    mark = mark.resize(scale, resample=Image.ANTIALIAS)

    if greyscale and mark.mode != 'LA':
        mark = mark.convert('LA')

    rotation = determine_rotation(rotation, mark)
    if rotation != 0:
        # give some leeway for rotation overlapping
        new_w = int(mark.size[0] * 1.5)
        new_h = int(mark.size[1] * 1.5)

        new_mark = Image.new('RGBA', (new_w, new_h), (0,0,0,0))

        # center the watermark in the newly resized image
        new_l = int((new_w - mark.size[0]) / 2)
        new_t = int((new_h - mark.size[1]) / 2)
        new_mark.paste(mark, (new_l, new_t))

        mark = new_mark.rotate(rotation)

    position = determine_position(position, img, mark)

    if img.mode != 'RGBA':
        img = img.convert('RGBA')

    # make sure we have a tuple for a position now
    assert isinstance(position, tuple), 'Invalid position "%s"!' % position

    # create a transparent layer the size of the image and draw the
    # watermark in that layer.
    layer = Image.new('RGBA', img.size, (0,0,0,0))
    if tile:
        first_y = int(position[1] % mark.size[1] - mark.size[1])
        first_x = int(position[0] % mark.size[0] - mark.size[0])

        for y in range(first_y, img.size[1], mark.size[1]):
            for x in range(first_x, img.size[0], mark.size[0]):
                layer.paste(mark, (x, y))
    else:
        layer.paste(mark, position)

    # composite the watermark with the layer
    return Image.composite(layer, img, layer)