There’s not much that can go wrong here, but when it does, it really goes wrong.
You probably changed a photo file’s name in the Finder—in the iPhoto Library folder, behind the program’s back. iPhoto hates this! Only grief can follow.
Sometimes, too, a corrupted picture file will also make iPhoto crash when you try to edit it. Open the file in another graphics program, use its File → Save As command to replace the corrupted picture file, and then try again in iPhoto.
Choose File → Preferences. Make sure that the Other button is selected and that a graphics program’s name appears next to it. (If not, click Other, then click Set, and then choose the program you want to use.)
Also make sure that your external editing program still exists. You might have upgraded to a newer version of that program, one whose file name is slightly different from the version you originally specified in iPhoto.
Most digital cameras produce photos in a 4:3 width-to-height ratio. Unfortunately, those dimensions don’t fit squarely into any of the standard print sizes.
The solution: Crop the photos first, using the appropriate print size in the Constrain pop-up menu (see page 142).
Highlight the file’s thumbnail and then choose File → Revert to Original. ...