A script to show a picture with grid

grid.py

import PIL.Image
import sys

filename = sys.argv[1]
im = PIL.Image.open(filename)
im = im.convert('L')
w, h = im.size
gs = min(w, h) // 12

for r in range(h // gs):
    for c in range(w):
        coor = (c, r * gs)
        im.putpixel(coor, 255-im.getpixel(coor))

for r in range(h):
    for c in range(w // gs):
        coor = (c * gs, r)
        im.putpixel(coor, 255-im.getpixel(coor))

im.show()