Returns the custom image associated with a given app. If there are
  multiple candidate images on disk, one is chosen arbitrarily.

def get_custom_image(user_context, app_id):
  """Returns the custom image associated with a given app. If there are
  multiple candidate images on disk, one is chosen arbitrarily."""
  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]