Earlier quoted context omitted.
The opposite sides of a die add up to seven. So the opposite side of the picture just looks like the negative of the image.
I am not following your train of thought. Care to elaborate a little?
If you're looking at a die, and see a 1, the opposite side will be a 6. If you're looking at a 2, the opposite side is a 5, and so on with 3 and 4.
So the back side of that portrait would have sixes in place of ones, and so on - generating a negative, with black taking the place of white, and white taking the place of black.