By default, banner photos will be inserted with the entire photo showing. You may want to cut out a part of the photo. For example, if there is a lot of sky showing in a photo you may want to remove some of the sky.
To reposition the photo, just place your mouse cursor over the photo so it gets the 4 headed arrow and drag to move it around. If your mouse is close to the page heading, the 4 headed arrow may not appear, and the photo may not move. You'll need to move the mouse cursor higher, so it is away from the heading.