How to change lock screen background Ubuntu Mate

I would like to change the lock screen background in Ubuntu Mate 16.04. I have found out how to change the background for unity by changing the com.canonical.unity-greeter setting but I can not find the corresponding setting for Mate 16.04. I'm not sure if this is a problem in 15.10 as well but I could find no reference to it. I spent some time going through different settings through gsettings to no avail.

for Ubuntu MATE 16.04 try the following:

1. Open a terminal ( [Ctrl] + [Alt] + [T] )
2. Change to the following directory:
   cd /usr/share/backgrounds/ubuntu-mate-common/
3. Rename the old background picture:
   sudo mv Ubuntu-Mate-Cold.jpg Ubuntu-Mate-Cold.old
4. Copy your image here:
   sudo cp [path/to/your/image] Ubuntu-Mate-Cold.jpg

Note: Do not change the file extension for the background image, even if it is not a jpg but for example a PNG. The background image should be displayed normally anyway.

To change the login screen background, do the following:

1. Open a terminal (Ctrl+Alt+T) and run sudo -H gedit /etc/lightdm/lightdm-gtk-greeter-ubuntu-mate.conf.
2. In the Gedit window that opens, find the line beginning with background =.
3. Delete whatever comes after the = and enter the path to the image you want to use.
   • If that path has spaces or any special characters ((,),-,_,*,etc), you will need to put a backslash (\) before every special character for it to work.
4. Once you've gotten the path entered, save the file (Ctrl+S) and close Gedit.
5. Reboot and you should see your background changed.

Source: https://ubuntu-mate.community/t/how-to-change-login-screen-background/216

Apparently the above does not change the lock screen background, and a different method is needed. I found the procedure here.

1. Open a file browser as root (sudo -H nautilus) and navigate to /usr/share/backgrounds/mate/desktop/.
2. Rename the Stripes.jpg file and the TwoWings.jpg file to .Stripes.jpg and .TwoWings.jpg.
3. Now, copy the image you want as the lock screen background into that folder.
   • Make sure the image is a JPG file.
4. Right click your image and select the "Make Link" option.
5. Rename the created link to Stripes.jpg.

Reboot and the lock screen should be changed.

IMPORTANT: There is a much easier way to change the background! Just go to LightDM GTK+ Greeter Settings in the control centre (find it through the system tab) and all the settings for the greeter (login screen) can be found there. Below is my previous response which will still work (if you want to use it)

If you are having trouble with the first response, then here is how I did it:
1) Open Gedit as root

sudo su 

Then type your password and then:

gedit

2) Click file, open, then when the box comes up, click 'other location'

3) Then click 'computer', then navigate to /etc/lightdm/

4) In this directory you should see a few folders and some config files. Now navigate to the 'lightdm-gtk-greeter-conf.d' directory and you should see a file similar to '99_ubuntu-mate.conf'. For me, this was the right config file for the greeter, or login screen.

5) Open the file and then you want to go to the line [greeter], where the commenting ends. Below that line you should see a line labelled 'background = /usr/share/backgrounds/ubuntu-mate-common/Ubuntu-Mate-Cold-lightdm.jpg'. This is the line you want to edit. Replace the original file with any jpg image of your choosing, save, and upon reboot your login screen background image should be changed. If you wish to replace the background with another stock Ubuntu Mate background, then the images are stored in /usr/share/backgrounds. (The only reason I changed my background was to get rid of the Mate logo which was halfway off my laptop screen!)

This is similar to the first response, but is a more graphical way of doing things, so should be easier for newer users.

NB - you do not need to put a backslash (escaping) before any special characters in your new background image file (at least I found that I didn't need to, it may be different for you)