You can do many editing tasks in iPhoto, such as rotating and cropping a photo, changing a color photo to black and white, adjusting exposure and contrast, and reducing red-eye. If you want to make other changes to a photo, you can open it in another image-editing application, such as Adobe Photoshop.
To edit photos in another application:
Choose iPhoto > Preferences, click Advanced, and choose “In application” from the “Edit photo” pop-up menu.
In the Open dialog, select the application you want to use to edit photos, and then click Open.
In iPhoto, select a photo and click the Edit button.
The photo opens in the application you selected.
Edit the photo and save it. Don’t change the photo’s filename or file format.
You see your changes the next time you open the photo in iPhoto.
Important: Nondestructive editing does not apply to photos in your iPhoto library that you edit in a different application. You can, however, always revert to the original version of the photo that was imported into iPhoto.
For RAW-format photos: By default, when you edit a RAW-format photo in another application, iPhoto opens a JPEG-format copy of the photo. To open the original RAW-format file instead, choose iPhoto > Preferences, click Advanced, and then select “Use RAW when using external editor.” When you finish editing, you must save the edited version on your computer and reimport it into iPhoto.
Note: If you use Mac OS X Lion v10.7 (or later), and want to edit your photos in Preview, click Unlock if an alert appears in Preview. For more information, see Apple Support.