Sets the custom image for `app_id` to be the image located at
  `image_path`. If there already exists a custom image for `app_id` it will
  be deleted. Returns True is setting the image was successful.

def set_custom_image(user_context, app_id, image_path):
  """Sets the custom image for `app_id` to be the image located at
  `image_path`. If there already exists a custom image for `app_id` it will
  be deleted. Returns True is setting the image was successful."""
  if image_path is None:
    return False

  if not os.path.exists(image_path):
    return False

  (root, ext) = os.path.splitext(image_path)
  if not is_valid_extension(ext):
    # TODO: Maybe log that this happened?
    return False
  # If we don't remove the old image then theres no guarantee that Steam will
  # show our new image when it launches.
  if has_custom_image(user_context, app_id):
    img = get_custom_image(user_context, app_id)
    assert(img is not None)
    os.remove(img)
  
  # Set the new image
  parent_dir = paths.custom_images_directory(user_context)
  new_path = os.path.join(parent_dir, app_id + ext)
  shutil.copyfile(image_path, new_path)
  return True