There is a private folder located in the .Piktures folder. The dot (".") in front of the folder makes it invisible to most file managers and apps.
Inside this folder, all the secret photos and secret videos are located. Their names have been changed and the content is encrypted with a private key which is unique to your email address and your device.
The benefits of this implementation is the strong security. This implementation offers a much stronger security than a naive implementation with a .nomedia file added to a folder. The files are encrypted and cannot be decrypted without the key.
The drawbacks is that when the photos and videos are secured, they are actually encrypted on-the-fly and moved to a subfolder inside the .Piktures folder. The encryption and decryption is slower than a simple move.
However, this is the only way to ensure a truly Secure Drive.