I've wondered about opening the camera from the lock screen, which gives access to Photos images. But I just tried covering my camera and, while the camera app opens and lets me take pictures with the primary camera, it only gives me access to pictures taken on that session-- not to the entire Photos Library.
Of course, if I don't cover the front camera, it sees that it's me, and it lets me view the entire library.
As markwmsn says, you have to get through password/face/fingerprint protection to scroll through Photos' images, so no need for an extra block.