I have been using personal photos for wallpaper on both home and lock screens with no problem.
Two days ago I ran the latest samsung android update, I mention this as this is the only change I have made.
Now my wallpaper image loads full screen with no problem, and then a short time later then reduces in size to around 30% of the top left of the screen
I have tried downloading other images. Again full size no problem and then becoming smaller without warning or obvious trigger.