KeepSafe - Keep your entire photo library safely stored in an encrypted private cloud.
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.
XnView MP - XnView is a free software that allows you to view, resize and edit your images. It supports more than 500 different formats!
Hexlock - Security app locker keeps apps private with smart profiles for any situation
FastStone Image Viewer - FastStone Image Viewer is a fast, stable, user-friendly image browser, converter and editor.