Finding the brightness of an image using pygame

[Zman350x]

As part of my science fair, I need to order images by brightness, so I created this bit of code below:

#imports

import pygame
from pygame.locals import *
pygame.init()

#images to test for brightness

TESTS = [
pygame.image.load("T1-0.png"),
pygame.image.load("T1-1.png"),
pygame.image.load("T1-2.png"),
pygame.image.load("T1-3.png"),
pygame.image.load("T2-0.png"),
pygame.image.load("T2-1.png"),
pygame.image.load("T2-2.png"),
pygame.image.load("T2-3.png"),
pygame.image.load("T3-0.png"),
pygame.image.load("T3-1.png"),
pygame.image.load("T3-2.png"),
pygame.image.load("T3-3.png")]

#testing loop

for image in TESTS:

    #set up display

    screen = pygame.display.set_mode((image.get_width(),image.get_height()))
    screen.blit(image,(0,0))
    pygame.display.update()

    #reset value

    value = []

    #loop to test each pixel

    for x in range(image.get_width()):
        for y in range(image.get_height()):

            #find brightness

            color = screen.get_at((x, y))
            color = (color[0]+color[1]+color[2])/3

            #add brightness to the value of the image

            value.append(color)

    #average the value of the image and display it to user

    print(sum(value)/len(value))

#end program

pygame.quit()
quit()

The premise behind it is that it blits an image to the screen and then goes pixel by pixel and finds the "brightness". It does this by averaging the RGB values (A white pixel would return 255, while a black would return 0). It then puts all those values in a list and averages that out. For whatever reason, one image, which is visibly brighter than another, outputs a lower score. Why? Is there a bug in my code? Or do I need to scrap it and re-write the whole thing?

Please reply ASAP, the Science Fair is due Friday (I may or may not have procrastinated).