Have you tried logging in to a test user account (Post back if you need help.) to see if your issue follows you across account lines?

Have you trashed iPhoto's cache at /Users/Your short name/Library/Caches/com.apple.iPhoto/Cache.db?

If you find no joy with either, and if you're not seriously into trouble-shooting, I'll throw trashing and reinstalling iPhoto onto the table.

The new Great Equalizer is the SEND button.

In Memory of Harv: Those who can make you believe absurdities can make you commit atrocities. ~Voltaire