return a cv image for the thumbnail

def img(self):
        '''return a cv image for the thumbnail'''
        if self._img is not None:
            return self._img 
        self._img = cv2.cvtColor(self.original_img, cv2.COLOR_BGR2RGB)
        if self.border_width and self.border_colour is not None:
            cv2.rectangle(self._img, (0, 0), (self.width-1, self.height-1),
                          self.border_colour, self.border_width)
        return self._img