Sometimes, the banner photo will need its height adjusted. Really tall photos take up a lot of screen real estate, so it's often best to reduce the height of tall photos.
To adjust the height of a photo, find the bottom edge of the photo and click and drag up or down to make the image taller/shorter.
If all the image is showing, you won't be able to make the image taller. There is a minimum height (300 pixels at 1200 pixels wide) for the banner image, so you won't be able to make the image shorter if it is already at this height.