python How to create watermark in image using text

watermarker.py

#!/usr/bin/env python
import os
from PIL import Image, ImageDraw, ImageFont

def work(filename):
    # Open the original image
    main = Image.open("img/"+filename)

    # Create a new image for the watermark with an alpha layer (RGBA)
    #  the same size as the original image
    watermark = Image.new("RGBA", main.size)
    # Get an ImageDraw object so we can draw on the image
    waterdraw = ImageDraw.ImageDraw(watermark, "RGBA")
    # Place the text at (10, 10) in the upper left corner. Text will be white.

    font_path = "/System/Library/Fonts/Palatino.ttc"
    font = ImageFont.truetype(font_path, 100)

    im = Image.open("img/"+filename)
    width, height = im.size
    print height
    print width
    waterdraw.text((20, height-100), "www.coderevi.com", fill=(225,225,225,225), font=font)

    # Get the watermark image as grayscale and fade the image
    # See <http://www.pythonware.com/library/pil/handbook/image.htm#Image.point>
    #  for information on the point() function
    # Note that the second parameter we give to the min function determines
    #  how faded the image will be. That number is in the range [0, 256],
    #  where 0 is black and 256 is white. A good value for fading our white
    #  text is in the range [100, 200].
    watermask = watermark.convert("L").point(lambda x: min(x, 100))
    # Apply this mask to the watermark image, using the alpha filter to 
    #  make it transparent
    watermark.putalpha(watermask)

    # Paste the watermark (with alpha layer) onto the original image and save it
    main.paste(watermark, None, watermark)
    main.save("img/"+filename, "PNG")

if __name__ == '__main__':
    li = os.listdir('img')
    for f in li:
        if("gif" in f): continue
        print  f
        work(f)