code: def _get_thumbnail_filename(self, source, geometry_string, options):
          
          key = tokey(source.key, geometry_string, serialize(options))
          # make some subdirs
          path = '%s/%s/%s' % (key[:2], key[2:4], key)
          return '%s%s.%s' % (settings.THUMBNAIL_PREFIX, path, EXTENSIONS[options['format']])
docs: Computes the destination filename.