MacOS – Damaged and can’t be open app error message

Since you've never purchased it from the App Store, you cannot re-download iPhoto from the App Store (unless you buy it again).

The only way you get re-download rights to iLife on the App Store is if you either purchased it from the App Store or bought a Mac that does not come with a software re-install disc.

Try one of the following solutions:

• Quit the App Store and reboot your Mac. Most of the times, your problem will be solved.
• Try again in 15 or 30 minutes. Sometimes, as large as the server farms might be, they simply can't handle the overload in request. Maybe later, there will be an open slot for you
• Flush the cache of the App Store. Opeq Finder and use the "Go to" option to go to ~/Library/Caches/com.apple.appstore. Remove the file and make sure you've emptied your trash.

I hope one of these things will help you out.