Grises

import pygame, sys, os
from pygame.locals import *
from sys import argv
import Image

def pixel():
    ima = argv[1]
    im = Image.open(ima)
    imagen=im.load()
    ancho, alto = im.size
    for i in range(ancho):
        for j in range(alto):
            (r,g,b)= im.getpixel((i,j))
            pix = (r + g + b)/3
            imagen[i,j]=(pix, pix, pix)
    nueva= 'nueva.png'
    im.save(nueva)

    return nueva
def main():

    pygame.init()
    screen = pygame.display.set_mode((500, 300))
    pygame.display.set_caption('Escala de Grises')
    imagen = pixel()
    img = pygame.image.load(imagen)
    screen = pygame.display.get_surface()

    while True:

        for event in pygame.event.get():
            if event.type == pygame.QUIT:
                sys.exit()
        screen.blit(img, (0,0))
        pygame.display.update()

if __name__ == "__main__":
    main()