Viber is a free calling and instant messaging application and is very secure and safe to use. Pictures, messages, etc. all can be done and sent in a very secure manner with this app. The app uses end-to-end encryption for its chats, which means that only the sender and the receiver know what's in the messages. Not even Viber itself knows.
For further security options, open the app and go to "More" in the lower left corner.
From there, go to settings.
From Settings, go to "Privacy".
From the options available in the "Privacy" part, you can observe your own safety and modify your account or look at any other option regarding security and privacy.