Why not just encrypt your sd card? We all know that as soon as you take a photo all apps want picture and contact access, so there's little point in storing stuff online? N.B I am a against a little app phobic.
Or if this is for pics etx android has it built in "private mode" its in the drop down settings at the top
anything you put in there is only viable when you turn on private mode.
Probbsly the easiest solution as its built in, unlocks with thumb print can be set to turn back off if the screen is turned off etc