Optical Illusion?

[Louis VI the Fat]

Spoiler Alert, click show to read: 

Now I see what they're doing!! At last I figured it out. Bit difficult to explain, but when you see her from behind, lifing up her left leg, the image is identical to seeing her from upfront, lifting up her right leg. Etc. etc.

Thanks, Husar, you're a genius.

Edit: spoiler tags might be a good idea.