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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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]