Q:

How to change the Unity lockscreen wallpaper?

I know there's Ubuntu Tweak, but if I remember correctly, it was discontinued a while back.  Didn't wanna try using it on my laptop just in case that screws it up.

A:

This solution will change the lock-screen wallpaper and the logon wallpaper (greeter screen) at the same time.
Note: In Ubuntu 18.04 only the lock screen wallpaper is changed. A different technique will be required to change the login wallpaper.
gksu gedit /usr/share/glib-2.0/schemas/10_unity_greeter_background.gschema.override

Then add the lines below:
[com.canonical.unity-greeter]
draw-user-backgrounds=false
background='/home/user_name/Pictures/picture_name.png'

In my example change /home/user_name/Pictures/picture_name.png to whatever directory and file name you want to use. Note .jpg file formats are also accepted.
Next compile the schema using:
sudo glib-compile-schemas /usr/share/glib-2.0/schemas

The final step is to log-out or reboot.
This works in Ubuntu 16.04 but I haven't tested it in Ubuntu 16.10 please respond via comment if you know for sure. Thanks.

You can use Nautilus to view wallpaper images and right click to change login screen and lock screen wallpaper: (Nautilus can set desktop wallpaper. How can it set login and lock screen wallpaper?).
Sample screen:

