You can crop a photo to keep only the portion you like and remove the rest.
Important: Cropping a photo changes its appearance in the photo library and in every album, slideshow, and project (books, cards, and calendars). To edit a photo without changing it everywhere it appears, duplicate the photo and edit the duplicate.
To crop a photo:
Select the photo and click the Edit button.
If the Quick Fixes tab (at the top of the Edit pane) isn’t selected, click it.
Click the Crop button.
Drag the selection window to enclose the area you want.
To crop to specific dimensions, select the Constrain checkbox, and then choose a size from the pop-up menu.
If you plan to use the photo in an iPhoto book, choose 4 x 3 (Book).
If you plan to use the photo as your desktop picture, choose Display to fit the photo to your computer screen.
To crop the photo to custom dimensions, choose Custom from the pop-up menu, and then type the dimensions.
To start over, click Reset.
Note: In the Constrain pop-up menu, the first two dimensions are in pixels; the rest are width-by-length ratios.
Do one of the following:
To save your changes, click Done.
To leave crop view without saving your changes, press Escape (Esc).
To cancel all changes made during this edit session (in any of the Edit panes), click “Revert to Previous.”
Note: If you’ve made only one edit, the button is labeled “Revert to Original.”