Sort of a long way around the barn, but I believe you might be able to use NoteEverything to accomplish what you're trying to do. The app allows encryption of a "photonote" so it can't be accessed without a password/pin.
I'm sure there are other apps that allow you to put a lock on a particular file once you know the filename. I'd suggest you download Astro file manager (or some equivalent) and see if you can achieve your objective by identifying the photos and storing copies in some specific folder where you can protect them.