By trying the answer of Senio I had no luck. But with little modification I hit the jackpot :

sudo -i
xhost +SI:localuser:lightdm
su lightdm -s /bin/bash
gsettings set com.canonical.unity-greeter draw-user-backgrounds 'true'
gsettings set com.canonical.unity-greeter background 'path-to-image'
exit

path-to-image is actually the path where the image you want as log in screen wallpaper is stored e.g. : /usr/share/backgrounds/x.jpg

I tested it and it's working

I found this page which offers some tips regarding this problem : Ubuntu13.04 LighDM

Cheap and dirty solution: Got to Terminal, then:

sudo nautilus

Then use Nautilus to copy your preferred background to

/usr/share/backgrounds/

Then rename the image that is being used to backup-imagenameold.png Then rename your preferred image to the filename of the original background.

I know it's not clean or lean, but it does the job asap ;)

Cheers, Rüssel

Its very simple.

1. Open a terminal

2. become root and change current folder

   sudo su
   cd /usr/share/backgrounds/
   

3. Copy the picture file to this location

4. Change the file name to warty-final-ubuntu.png

That will change the login screen background image.

Source

Hmmm .. I try install dconf-editor to change that background. Open Terminal and type this script ..

sudo apt-get install dconf-editor

Run dconf-editor:

sudo dconf-editor

And show the Window ... Open com > canonical > unity-greeter Then change:

draw-user-backgrounds: 'true'
background: 'path-to-image'

I believe what you are looking for is:

sudo apt-get install lightdm-gtk-greeter
sudo vim /etc/lightdm/lightdm-gtk-greeter.conf

Change the line:

background=/usr/share/backgrounds/warty-final-ubuntu.png

to whatever you want. Hope this helps

Canonical added the support for it in Ubuntu 16.04. Take a look at this link -> https://help.ubuntu.com/16.04/ubuntu-help/user-changepicture.html

I accidentally have found the solution which works great. It even preserves animation of emerging wallpaper during login screen (which i wanted the most). Step by step what i did:

1. (lightDM) Change the wallpaper login screen to default. Type in Terminal:

   sudo -i
   xhost +SI:localuser:lightdm
   su lightdm -s /bin/bash
   gsettings set com.canonical.unity-greeter draw-user-backgrounds 'true'
   gsettings set com.canonical.unity-greeter background ''
   exit
   

   This will change logon wallpaper to default. (We need it for animation thing)

2. Make a startup program named for example 'WallpaperChange', which executes this line:

   gsettings set org.gnome.desktop.background picture-uri \
             file:///usr/share/backgrounds/ur_desktop_wallpaper_here.jpg
   

   During every startup it changes your desktop background on ur_desktop_wallpaper_here.jpg

   Note: Whole path to ur_desktop_wallpaper_here may be changed.

3. The most important thing. Change your desktop background via GUI in Preferences Options on the wallpaper, which you want to have on logon screen.

4. Restart your computer.

5. Uncheck startup program 'WallpaperChange'. You don't need it any more now. During the next change of the wallpaper you will use it again.

I had only tested it on ubuntu 13.04 and found a solution by accident. I cannot provide you a technical answer why it works. It may depend on what you had done with your computer before setting logon wallpaper. I will reinstall ubuntu, try this code again and learn if my solution needs any further editing. (Added 4th line in p.1)

Greets.

I am using Ubuntu 14.04 (all Desktop settings are standard and I am using the standard shell) and I think it is even simpler than all solutions stated above.

The only thing I do, is downloading a picture or getting one of my own photos, fitting with the screen size, click the right mouse button on the downloaded picture and select "Set as background".

This process seems to copy the image into the users Picture/Wallpaper folder and, given the right size of the image, will show this picture also at next login.

I got the simpler way in Ubuntu 14.04 to solve this problem. You just have to change the permissions of image by typing command in terminal as follows.

chmod 644 'path-of-file-to-change-permission'

Replace path-of-file-to-change-permission with the path of your picture.

For example your background images are in /home/Admin/Pictures then type command as follows.

chmod 644 /home/Admin/Pictures/*.jpg

This will work.

Ubuntu 18.04 up to 19.10

You need to change the path to your background image in the default theme /usr/share/gnome-shell/theme/ubuntu.css

1. First backup the file!!!

   cp /usr/share/gnome-shell/theme/ubuntu.css /var/tmp/
   

   In case you have a typo somewhere, you won't be able to access the login screen itself. If you make any mistake, then you have to fall-back to the terminal to revert the change or make a correction.

2. Change the background path in the id #lockDialogGroup

   Copy the current image to an unencrypted folder (in case the current image resides inside an encrypted folder) with

   cp "$(gsettings get org.gnome.desktop.background picture-uri|sed "s~'file://\(.*\)'~\1~")" /usr/share/backgrounds/current-background.png
   

   And add this in the #lockDialogGroup section, for example:

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

3. Maybe restart the computer to see the effect.

see https://askubuntu.com/a/1071765/34298

for Ubuntu 20.04 see How do I change login screen theme or background in Ubuntu 20.04?