Change picture on Lock screen and/or Home Screen (Wallpaper and style): Touch and hold on an empty space on a Home screen to reveal edit mode then tap on ‘Wallpaper and style’ to open this area in Settings (or go to Settings ❯ Wallpaper and style) tap ‘Change wallpapers’ to see the options (e.g. use a photo in Gallery, select one then follow the steps to add to Lock screen and/or Home screen)