from PIL import Image
img = Image.open('C:\pics\mona_lisa.jpg')
pixels = img.load()
for i in range(img.width):
    for j in range(img.height):
        r, g, b = pixels[i, j]
        s = (r + g + b) // 3
        pixels[i, j] = (s, s, s)
img.show()