code: def _thumbnail_local(self, original_filename, thumb_filename,
                           thumb_size, thumb_url, crop=None, bg=None,
                           quality=85):
          
  
          # create folders
          self._get_path(thumb_filename)
  
          thumb_url_full = url_for('static', filename=thumb_url)
  
          # Return the thumbnail URL now if it already exists locally
          if os.path.exists(thumb_filename):
              return thumb_url_full
  
          try:
              image = Image.open(original_filename)
          except IOError:
              return ''
  
          img = self._thumbnail_resize(image, thumb_size, crop=crop, bg=bg)
  
          img.save(thumb_filename, image.format, quality=quality)
  
          return thumb_url_full
docs: Finds or creates a thumbnail for the specified image on the local filesystem.