If a photo does not fit the frame perfectly, the frame automatically crops the photo to fill the entire display.
The cropping will be centered around what the sender selected as the important part. This is just to ensure that the selected part won't be cropped out under any circumstances.
Example
The entire photo will always be sent to the frame regardless of the selected part, and should you prefer to display the photo in its full size, you can just follow the steps in this quick guide: Why Is My Photo Cropped?