Is there an iPhone app that allows you to block photos?

I've tried a few and they're all complicated. They make you transport all your pictures into an app instead of locking the photos on the iPhone. I want it to lock the photos as soon as you click on the photo app. I want it to ask for a passcode to access the pictures ON the phone itself, NOT through an app. Like a feature or something?
My sister always goes into my pictures when she uses my phone to talk to my dad and sees my private pictures.

Http://download.cnet.com/8301-2007_4-20120804-12/lock-up-your-photos-on-ios/