Lock Screen Wallpaper Location
That depends on your Android version, but only changes a little. Wherever it is, you need root-access to retrieve it.

For Android 6 and under:

/data/system/users/0/lock_wallpaper


While primary (mainscreen) wallpaper is available at /data/system/users/0/wallpaper.

For Android 7+, the file name has changed to wallpaper_lock and is still available at the same place. You may want to fetch wallpaper_lock_orig instead, however.