docs: def prep_image(image, tile_size):
      """Takes an image and a tile size and returns a possibly cropped version
      of the image that is evenly divisible in both dimensions by the tile size.
      """
      w, h = image.size
      x_tiles = w / tile_size  # floor division
      y_tiles = h / tile_size
      new_w = x_tiles * tile_size
      new_h = y_tiles * tile_size
      if new_w == w and new_h == h:
          return image
      else:
          crop_bounds = (0, 0, new_w, new_h)
          return image.crop(crop_bounds)