return a cv image for the icon

def img(self):
        '''return a cv image for the icon'''
        SlipThumbnail.img(self)

        if self.rotation:
            # rotate the image
            mat = cv2.getRotationMatrix2D((self.height//2, self.width//2), -self.rotation, 1.0)
            self._rotated = cv2.warpAffine(self._img, mat, (self.height, self.width))
        else:
            self._rotated = self._img
        return self._rotated