code: def get(img, cache_dir=CACHE_DIR, iterative=False):
      
      if os.path.isfile(img):
          wal_img = img
  
      elif os.path.isdir(img):
          if iterative:
              wal_img = get_next_image(img)
  
          else:
              wal_img = get_random_image(img)
  
      else:
          logging.error("No valid image file found.")
          sys.exit(1)
  
      wal_img = os.path.abspath(wal_img)
  
      # Cache the image file path.
      util.save_file(wal_img, os.path.join(cache_dir, "wal"))
  
      logging.info("Using image \033[1;37m%s\033[0m.", os.path.basename(wal_img))
      return wal_img
docs: Validate image input.