The photo gallery and message archive are arguably two of the most private places on an Android smartphone or tablet that need to be tightly secured. Restricting access to these items allows you to avoid the prying eyes of those who borrow your device, or worse, in the event that your phone is stolen or falls into the wrong hands.
There are a variety of ways to lock down access to photos, text messages, and other areas of your phone that you want to keep private. Let's take a look at a few simple methods.
Using AppLock application
Whether you want to lock down access to your photo gallery, messaging app, or any other app on your Android device, AppLock can do it. AppLock doesn’t go as far as locking down individual messages or photos, but it can secure the entire app so that when someone wants to access it, they’ll need to provide the exact security code you set up beforehand.

AppLock is completely free and very easy to use. You hardly have to spend time getting used to the application because everything is displayed clearly, intuitively, and well integrated with the operating system. Besides the traditional password, AppLock also allows you to set a pattern to lock a specific application.

Along with app security, AppLock also comes with quite a few other features, such as system cleaning. But it's best to only grant it the permissions necessary to secure your apps.
Use Telegram, Signal, or other secure messaging apps
It's a sad fact that WhatsApp and Messenger - two of the most popular online messaging apps in the world today - are pretty bad when it comes to security and privacy. Not only are they tied to Facebook, but they also lack many essential privacy features.

Instead, consider switching to highly regarded secure messaging apps like Telegram and Signal. Both chat apps promise to offer more privacy controls than WhatsApp, Facebook Messenger, and other popular messaging apps. These include built-in features that let you set a passcode for the app. Signal also lets you block screenshots from appearing in your recent contacts. Both apps support “self-destructing messages,” which automatically disappear after a set amount of time for maximum privacy.
Secure Folder (for Samsung devices only)
Secure Folder is an extremely useful security feature that many Samsung smartphone users do not even know about and take advantage of to protect their privacy. Secure Folder is a tool only available on Samsung Android devices, designed to allow users to encrypt, set passwords and biometric locks for all applications and data on their devices.

In particular, Secure Folder is also integrated with Samsung's famous Knox security technology. Therefore, your application will be protected not only from unauthorized local access situations, but also from online threats.