I've had my hard drive replaced and the iPhoto app is gone

If you replace a hard drive nothing is still there -

reinstall from the original source

Could be

1 - the restore disks that came with the computer

or

2 - the iLife DVD that you purchased

or

3 - the app store - log in under the Apple id used to purchase the software or to set up the computer - look under purchases - you might have to unhide or accept the Application first

LN

iPhoto is not part of OS X. It's part of iLife. You can re-download it from the App Store under Purchases.