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.
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.