Depends on your device and configuration. On any device you could swipe to camera, tap the photo thumbnail, hit "all photos " and you'll be prompted for touchID or passcode if you have a passcode, or taken straight to all photos if not. At that point you're unlocked in photos.app but you wouldn't be able to get to the homescreen unless you're on an ipad with gestures enabled. At least there's no way I can think of or see. Someone else might have an idea.