To protect your photos with DFNDR's Private Gallery, open the app, tap on TOOLS tab and select the Vault feature.
Enter your pattern (or numerical password).
Tap on "ADD PHOTOS" (green button at the bottom of the screen).
Choose an album.
Select the photos you want to hide and tap on "PROTECT PHOTOS".
REMEMBER: If you uninstall DFNDR for any reason, your Private Gallery and the photos within it will not be saved so make sure you save ahead of time should you decide to uninstall.