Photo Pinner solves that, using the inbuilt features of Android 5.0+
Simply select a photo in your favourite gallery app, share it to Photo Pinner and press the pin button. Android will take care of the rest - you can even make it go to the lockscreen before unpinning!
Photo Pinner is totally free, free from ads, has a tiny file size and needs no permissions
It also supports basic pinch zoom features, and views in fullscreen