Most photo editing problems stem from using another program to edit the photos.
Photos Don't Open in an External Program
Although it is unlikely that you'll run into this problem, it could be frustrating. Here are a few reasons it could happen:
iPhoto Crashes When You Double-Click Photos
There are a few reasons iPhoto might crash when you double-click a photo to edit it within iPhoto:
iPhoto Doesn't Allow Editing
Some people have reported importing images from a CD that they later couldn't edit in iPhoto. As a workaround, convert the images in GraphicConverter from JPEG to TIFF, for instance, and see if you can import and then edit those versions.
Also verify that the permissions on the files allow your user to write to the files. To check, select them in the Finder, choose Get Info from the File menu (), and look in the Ownership & Permission area.
Revert to Original Dimmed
You may see the Revert to Original command in the File menu dimmed after you've made a change. This can happen if you drag a photo from iPhoto to another program to edit the photo. If you do that, iPhoto will be unable to track changes you've made and Revert to Original will be dimmed. There's no workaround, other than making sure to open photos for editing in external applications properly from within iPhoto.
Thumbnails Are Corrupted or Cause Crashes
If a thumbnail doesn't reflect edits, is entirely black, or causes iPhoto to crash, hold down while clicking the iPhoto icon in the Dock to launch it. In the Rebuild Photo Library dialog, select the first two checkboxes to rebuild all thumbnails.
Beware of Too Much Editing
One quick warning about editing. iPhoto saves edited photos using the lossy JPEG compression format, which throws away information in the image to keep file sizes small. That's no problem in normal usage, but an excessive number of edits could cause a slight degradation in image quality.