code: def get_custom_image(user_context, app_id):
    
    possible_paths = _valid_custom_image_paths(user_context, app_id)
    existing_images = filter(os.path.exists, possible_paths)
    if len(existing_images) > 0:
      return existing_images[0]
docs: Returns the custom image associated with a given app. If there are
    multiple candidate images on disk, one is chosen arbitrarily.