Can't rotate cubemap's texture

It seems to be a very stupid question but it did bother me!
I generate a cubemap using 6 texture images like this way:

skybox_texture = self.loader.loadCubeMap(os.path.join(self.bullet_path, ‘background#.jpg’))

However, one face’s orientation is wrong. Then I just simply rotate the according image using an image editor. The weird thing is that no matter how I rotate the image (and of course save the change), the rendered sky box always show the wrong orientation of that face.

It is quite abnormal and anyone can tell me which part goes wrong?

Have you read this section?