This app was mentioned in 10 comments, with an average of 1.30 upvotes
I've made this app once for this reason, it allows you to show a preselected set of images to others and protects your complete phone, others cannot leave the image viewer without a password or whatever protection mode you choose.
No home button and no back button works so it's save to give away the phone.
It's very easy to use. Use the gallery of your choice, select images and share them with my app, that's it.
https://play.google.com/store/apps/details?id=com.michaelflisar.privacyimageviewer
I just published such an app... It allows to show single/multiple pictures and works with ANY photo app. Just share your selection with the app and you can safely give your phone away to others...
https://play.google.com/store/apps/details?id=com.michaelflisar.privacyimageviewer
Link me: SavePic
App SavePic is what you want. Not another gallery app.
Edit: bot got wrong app. This is the one: https://play.google.com/store/apps/details?id=com.michaelflisar.privacyimageviewer
What I meant is following:
https://play.google.com/store/apps/details?id=com.michaelflisar.privacyimageviewer
PS: a real privacy image viewer (that CAN'T even be exited) can be found here: https://play.google.com/store/apps/details?id=com.michaelflisar.privacyimageviewer