Pygame PNG image looks corrupt

I'm following this guide to try and display a basic PNG image inside a Pygame window. My image is a simple 150x150 green ball with no transparency called ball.png, located in the same directory as my Python script:

However, when I start my program, all that appears is a glitchy image of the correct size and location:

I am using this code to display the image (identical to the code given in the tutorial linked above):

import pygame
import os

_image_library = {}
def get_image(path):
        global _image_library
        image = _image_library.get(path)
        if image == None:
                canonicalized_path = path.replace('/', os.sep).replace('\\', os.sep)
                image = pygame.image.load(canonicalized_path)
                _image_library[path] = image
        return image

pygame.init()
screen = pygame.display.set_mode((400, 300))
done = False
clock = pygame.time.Clock()

while not done:
        for event in pygame.event.get():
                if event.type == pygame.QUIT:
                        done = True
        screen.fill((255, 255, 255))
        screen.blit(get_image('ball.png'), (20, 20))
        pygame.display.flip()
        clock.tick(60)

Why would this happen? Is there a flaw in the code? I have tried replacing the PNG with different ones, but that just changes how the glitchy image looks - the image is still not displayed correctly. I'm using OS X 10.11 El Capitan, Python 3.5.0 and Pygame 1.9.2.