Over time, iPhoto can become less efficient at accessing the photos in your library. Rebuilding the cache can help iPhoto display your photos faster, and can also resolve issues such as thumbnails that appear to be dimmed or missing.
After upgrading iPhoto, you might see a message asking if you want to rebuild your thumbnail cache. You can choose whether to do it then or come back to it later.
Important: You should back up your iPhoto library before rebuilding your thumbnail cache.
To rebuild your thumbnail cache:
If iPhoto is open, quit it.
Hold down the Option and Command keys while you open iPhoto (click the iPhoto icon in the Dock).
The Rebuild Photo Library window opens.
Select at least these two checkboxes:
Rebuild the photos’ small thumbnails: This option rebuilds only the small versions of the thumbnails, such as those that appear in the photo browser when you are creating a book or slideshow.
Rebuild all of the photos’ thumbnails: This option rebuilds all the thumbnails associated with a photo, including the small ones and the thumbnails that appear in a photo album or an Event.
For information about the remaining checkboxes, visit the iPhoto Support website and search for “rebuilding the iPhoto library.”
Rebuilding the thumbnail cache may take several minutes.