Photos for the homepage and interior page banners will fill the entire space available for the image in each design theme while maintaining its aspect ratio. As a result, some cropping may occur. For example, if you upload a very wide photo, some of the left and right side of the image may not be visible. If it's too tall, some of the top and bottom of the image may be cropped off.
If you want your entire photo to be visible, you'll need to resize it to the dimensions used by the design theme which are provided next to the upload field for each image on the Theme Preferences page (Design ⇨ Theme Preferences).