I have been trying to change my login screen wallpaper to one of my own using ubuntu-tweak. The wallpapers are located in Ubuntu's default wallpaper folder and I have given all users read privileges on them.

Now there isn't even a wallpaper left on the login screen.

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Have a look at my answer Here, and see if it helps. –  Mitch Jun 27 '13 at 20:12
I have already tried that and it didn't work. –  Joren Jun 27 '13 at 20:30
What version of Ubuntu do you have? I use to have a login screen with the wallpaper of the user but now in Ubuntu 13.04 this function is disabled, at least by default.. –  Lucio Jun 27 '13 at 22:13
@Lucio I'm running Ubuntu 13.04. Would there be an alternative option? –  Joren Jun 28 '13 at 13:36

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

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And don't forget to give your wallpaper sufficient permissions ;) –  Joren Oct 7 '13 at 15:15
I also confirmed that this works in 13.10 too. –  Joren Oct 17 '13 at 21:33
Works in 14.04 as well, thanks! –  Greg Kramida Jun 3 '14 at 13:43

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.

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.

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I have followed your steps but I am still seeing no wallpaper in my login screen. –  Joren Jul 21 '13 at 14:52

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

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Its very simple.

1. Open a terminal, (become root)

cd /usr/share/backgrounds/

2. Copy the picture file to this location

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

That will change the login screen :)

Thanks to annamalai.

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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'

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In the past this would work for me. Im am not sure if this will work in a unity environment.

Run this.

sudo cp /usr/share/applications/gnome-appearance-properties.desktop /usr/share/gdm/autostart/LoginWindow


sudo unlink /usr/share/gdm/autostart/LoginWindow/gnome-appearance-properties.desktop