ibooks author - resize cover

I am preparing a comic book via ibooks author. Several comics I have downloaded feature a cover that is the direct aspect ratio of the original comic (so it is slimmer, narrower than the template size) I will attach a screenshot:

The Proof is my own, the samples are somehow resized, when opened they are not against a white background, but cropped...

Does anyone know how this was accomplished? Much thanks if you can help!