def get_image_url(image_url, target_size='original'):
    """Given an image URL (of any size), return the URL for the specified size"""
    if not str(image_url).startswith('http'):
        return None
    for size in ('square', 'small', 'medium', 'large', 'original'):
        image_url = image_url.replace(size, target_size)
    return image_url