iPhoto problems. HELP!

How are you trying to download?

Did you follow these instructions?

See: Downloading past purchases from the App Store, iBookstore, and iTunes Store

Have you tried downloading it from here:

Open the App Store app > Updates > Purchased

And what happens?

It should be possible directly on your device: From the link I poined to:

1. Tap App Store.
2. Make sure you're signed in with the same Apple ID used for the original purchase.
3. Tap Updates.
   
4. Tap Purchased on the resulting screen.
   
5. Locate the app in your Purchased tab.
6. Tap the download icon.
   
7. The app will begin downloading and you'll be taken back to your home screen.

Where does this not work?

Try this:
Quit all apps: Double tap the Home button and swipe the app preview page up

and

Reset: Hold the Sleep/Wake and Home buttons and don’t let go until the screen goes dark and the Apple logo appears (no data will be lost)

In addition to quitting and resetting check this list:

iOS: Troubleshooting applications purchased from the App Store

If resetting does not help, back up and restore: http://support.apple.com/kb/HT1414