from apple:
1. In iPhoto, select a photo to crop from your photo library.
2. Click the Edit button below the photo viewing area.
3. Choose the dimensions for your photo from the Constrain pop-up menu. For example, if you plan to include the photo in a photo book, you can choose "4 x 3 (Book, DVD)." You can also choose None and select the crop area freely.
4. Notice how iPhoto shows you the area of the photo that will be cropped. If you don’t like the crop area, click outside the cropped area and then place the pointer where you want the cropped photo to begin.
5. To move the cropped area, place the pointer inside the cropped area and drag back and forth until it is positioned exactly where you want it.
6. Drag the pointer diagonally to select the crop area. Notice that if you’ve set a size constraint, you can select only the part of the photo that will fit in the size you specified.
7. When you are ready, click the Crop button, which is next to the Constrain pop-up menu.
8. When you’re done, you can save your changes by clicking the Done button.
