There is bunch of wallpaper applications where you can change lock screen wallpaper separate from main wallpaper. If I am not mistaken from API 24 it is possible to do such kind of thing, but I find very few articles related to this besides they are TOO old.
WallpaperManager has a number of methods to help you get or set the specific wallpapers. As you want the lock screen wallpaper, use FLAG_LOCK. Relevant methods include:
setBitmap(Bitmap fullImage, Rect visibleCropHint, boolean allowBackup, int which)