How can I secure my pictures, videos and other files stored on my phone?
I would recommend using full disk encryption and keeping your bootlaoder locked and phone unrooted if you want the best possible encryption that Android natively provides.

This article explains how to do a full system encryption on Android.
http://www.howtogeek.com/141953/how-to-encrypt-your-android-phone-and-why-you-might-want-to/