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 = cv.CreateMat(2, 3, cv.CV_32FC1)
            cv.GetRotationMatrix2D((self.width/2,self.height/2),
                                   -self.rotation, 1.0, mat)
            self._rotated = cv.CloneImage(self._img)
            cv.WarpAffine(self._img, self._rotated, mat)
        else:
            self._rotated = self._img
        return self._rotated