I hoped you do not notice it (I only noticed when I was done).
Actually, when you are with the simple, rectangular shape first, you can right click on the face > Texture > Position and the Flip it horizontally (left/right). This can be done separately on either side but be aware that some rendering applications will not be able to render these different maps (they generally render whatever is on the front face).
The reason I ended up with this is that I imported the image as an image entity and exploded afterwards. In this case, SU creates a face with the image projected on it - and the mirror on the other side (it has the reasons we can go into later if you want).