Lock My Photos offers more functionality than most apps in the category, letting you passcode-protect your images, and also adds several more-unusual things you can do from within the app. To get started, you'll need to create a four-digit passcode using the app's number pad. Once you're in the default folder, you'll be able to add photos from your iPhone library, or snap new pictures you want to keep private.
Lock My Photos offers a few more features than just a locked folder. You also can view your images as a slideshow, and even add transitions, adjust the time between slides, and shuffle the images in the slideshow--all from within the settings.
Perhaps the best feature of all in Lock My Photos, for those who need a hidden folder, is the Decoy passcode. You create the decoy in settings. Then, when you launch the app and enter the decoy passcode, the app opens another instance of the program where you can create folders of photos of a more...mundane variety. In other words, if someone demands that you enter your passcode, you'll be able to open the program without revealing your truly secret stash of images. It's also worth noting that, unlike HD Photo Lock, for example, when you put the app to sleep, then return, Lock My Photos requires that you enter your passcode to regain access.
Overall, Lock My Photos is a solid choice as a way to keep your hidden photos safe, with some added extras that make it even better. There is also a free version of this app in the iTunes App Store, but you'll only be able to secure 16 photos total.