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I'm working on one now called Optic that's in open beta: https://play.google.com/store/apps/details?id=com.dodgingpixels.gallery.beta
I'm beta testing a new gallery right now. It'll let you select a set of images to lock swiping to. It blocks back presses but not home.
Are you just trying to exclude certain folders from showing in the gallery? If so, a lot of third-party galleries will offer this.
I'm working on one now called Optic: https://play.google.com/store/apps/details?id=com.dodgingpixels.gallery.beta
I'm beta testing a new gallery app that has this feature.
try opticit is a new app still in beta phase with material design.
I think you need,
Optic Gallery - It has slideshow & swipe lock option.
Simple Gallery - Very basic app. Just change 'background colour for fullscreen media' - default colour is grey. Black background looks better when watching full screen media. Enable this dark background option from it's app settings.
If your grandma is having problem using android , You can simplify it using some apps. These apps are specially designed for old or non tech-savy people.
Koala phone launcher
Big launcher - It supports different themes.
These apps will really help your grandma.
Link me: Optic Gallery
Edit: Bots are sleeping it seems, so here's the link.
Optic Beta is my choice. It does everything I need it to and looks great.
Link: Optic Beta
You can share pictures to Optic from another gallery or file explorer to lock swiping.
My choice is Optic: Gallery App
I have tried Simple Gallery, Camera Roll etc.. But Optic is simple and yet awesome..
Optic Gallery has it
Here it is.