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 you can find it in your Purchases List.