To re-install iPhoto
1. Put the iPhoto.app in the trash (Drag it from your Applications Folder to the trash)
2a: On 10.5: Go to HD/Library/Receipts and remove any pkg file there with iPhoto in the name.
2b: On 10.6: Those receipts may be found as follows: In the Finder use the Go menu and select Go To Folder. In the resulting window type
2c: on 10.7 they're at
A Finder Window will open at that location and you can remove the iPhoto pkg files.
If you purchased an iLife Disk, then iPhoto is on it.
If iPhoto was installed on your Mac when you go it then it’s on the System Restore disks that came with your Mac. Insert the first one and opt to ‘Install Bundled Applications Only.
If you purchased it on the App Store or have a Recent Mac you can find it in your Purchases List.
I had this issue too with iPhoto 11. After being on the phone with Apple Support for about an hour trying different things we resolved by backing up my photos to an external hard disk.
Open Apps Store and select Purchases, make sure iPhoto shows there.
Open Finder and select Applications from the bar at the left of the screen, you should be able to see iPhoto in the list of applications. Click on iPhoto and from the bar at the top of that screen select the symbol which looks like a cog / wheel and from the drop down select Move To Trash.
Go back to App Store and click on Install at the side of iPhoto.
Once reinstalled it works fine.