Ok,
@Marilyn.... We take so many photos and not all are taken with the intention of making it into a a banner here, right? So from the lot we take, we have to choose the good ones which will have a small area of 1500x150 pixels. We have to choose the right rectangle in the larger whole rectangle photo originally taken. Not all areas of the photo are suitable for showing as a banner.
Did I explain it well enough?