How to change Ubuntu Budgie login screen background?

I have simple Ubuntu 18.04.1

In my opinion the best way to do this:

step1: make backup of the file /usr/share/gnome-shell/theme/ubuntu.css - if you have another desktop for ubuntu you can find file ubuntu.css just using command locate ==> locate ubuntu.css

step2: vim /usr/share/gnome-shell/theme/ubuntu.css and find part below

   #lockDialogGroup {
   background: #2c001e url(resource:///org/gnome/shell/theme/noise-texture.png);
   background-repeat: repeat; }

step3: change it to

   #lockDialogGroup {
   background: #2c001e url(file:///usr/share/backgrounds/warty-final-ubuntu.png);
   background-repeat: no-repeat;
   background-size: cover;
   background-position: center;
    }

clarification: as you can see, we changed
"background: #2c001e url(resource://" part to the ==> "background: #2c001e url(file://"
and then set up path for the image.
I used /usr/share/backgrounds/ for the path with image. Also I added some css properties to center the image.

This is only the css file configuration. That's it)

Open the Login Window app in the menu under Administration.

If it is missing from the menu then install it:

sudo apt install lightdm-settings

Enter your credentials when prompted. Click on the default background image icon, then navigate to /. Click on home>username>Pictures. Select your desired pic, click Open the in upper right corner, close the app. Log off and you will see your desktop background on the Login screen.

Thanks.

I tried all of those and none worked. This does, for sure:

As root user, just copy your_image.png to /usr/share/backgrounds and ....

# cd /usr/share/backgrounds
# cp warty-final-ubuntu.png warty-final-ubuntu.png.stock
# rm warty-final-ubuntu.png
# ln -s your_image.png warty-final-ubuntu.png
# reboot

...where your_image.png is the name of the actual image you want to display. The warty one is big: 4096 x 2304. I matched mine to the same size.