Step 4: Mouthing off...
At the cut, fold back both sides of the cut at a 45 degree angle. Crease back and forth to get a sharp fold.
Open the picture. Now push or pull out the point of the newly formed triangular peaks.
At the same time, close the picture so that you can kinda squish it so the peaks will be flattened out between the pages.
You have now created the pop-up part of the card.