code: def go_previous_thumbnail(self):
          
          if self.current_thumbnail is not None:
              index = self._thumbnails.index(self.current_thumbnail) - 1
              index = index if index >= 0 else len(self._thumbnails) - 1
              self.set_current_index(index)
              self.scroll_to_item(index)
docs: Select the thumbnail previous to the currently selected one.