F-Stop Media Gallery - Image and video organizer.
Photok - A safe place for your photos. It stores your photos encrypted on your device and hides them from others.
Camera Roll - Simple Gallery App for Android, with lovely Material Design.
Google Photos - All your photos are backed up safely, organized and labeled automatically, so you can find them fast, and share them how you like.
digiKam - Professional Photo Management with the Power of Open Source
Hide Secrets - Hide Secrets is an app that allows hiding everything they deem confidential on their smartphone.