iPhoto '11: Back up your iPhoto library
It’s a good idea to routinely back up, or archive, your iPhoto library in case image files become corrupted or are unintentionally erased from your hard disk.
If you have an external hard drive set up, you can use the Time Machine application included with OS X to back up your photos. After you set it up, Time Machine automatically makes regular backups of your computer, including your entire photo library.
To restore photos, quit iPhoto, open the folder containing your iPhoto library, and then open Time Machine.
To open Time Machine, click its icon in the Dock, or click the Time Machine icon in the menu bar and choose Enter Time Machine.
For more information on Time Machine, see the topics in Help Center.