The following code opens an image named "hopper" and creates a Draw object using it to then draw two lines. First, a line from the top left (0,0) to the bottom right. Then, a line from the top right to the bottom left.

import sys
from PIL import Image, ImageDraw

with Image.open("hopper.jpg") as im:

    draw = ImageDraw.Draw(im)
    draw.line((0, 0) + im.size, fill=128)
    draw.line((0, im.size[1], im.size[0], 0), fill=128)

    # write to stdout
    im.save(sys.stdout, "PNG")