Most important apps on iOS have a log in. That's basically the same as a lock. There is no app that locks other apps. So, you have the log ins, and you can use cloud based apps like Box and Dropbox to store photos and videos with a log in. Then delete them from the camera roll. Or use local photo album apps like MyPhoto. Upload your photos, delete from the camera roll, and lock the app with a key.
Then there's locking your phone. That's the best way to keep others from seeing anything. So there you go. No need for an app to lock other apps.