from PIL import Image

im = Image.open("BarredRock.jpg")
width, height = im.size

pix = im.load()
print pix[500, 500]
for i in range(width):
    for j in range(height):
        r,g,b = pix[i,j]
        pix[i, j] = (r, g * i/width, b * j/width)

im.show()