How do I set the LightDM wallpaper in Ubuntu 14.04?

In my case this Ask Ubuntu answer solved the problem.

The org.gnome.settings-daemon.plugins.background setting had been disabled but I could not determine why. Once I checked this checkbox in dconf-editor and changed my wallpaper, it was reflected on the login screen.

Dconf-Editor: ORG/GNOME/SETTINGS-DAEMON/PLUGINS/BACKGROUND

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We can get the value of the above setting by running this command:

gsettings get org.gnome.settings-daemon.plugins.background active

We can change this value with the command:

gsettings set org.gnome.settings-daemon.plugins.background active 'true'

How to Change the LightDM wallpaper

1. Install Ubuntu Tweak

   First Install Ubuntu Tweak Click here to download the .deb and install it though Gdebi, Ubuntu Software Center, ect... Or You can install it though the terminal by these two/three commands:

   wget https://launchpad.net/ubuntu-tweak/0.8.x/0.8.7/+download/ubuntu-tweak_0.8.7-1%7Etrusty2_all.deb
   sudo dpkg -i ubuntu-tweak_0.8.7-1%7Etrusty2_all.deb
   

   OR

   sudo add-apt-repository ppa:tualatrix/ppa
   sudo apt-get update
   sudo apt-get install ubuntu-tweak-tool
   

2. Open Ubuntu Tweak

   and follow the steps shown in the images below:

If you're going to change a file that belongs to a package (/usr/share/backgrounds/warty-final-ubuntu.png belongs to ubuntu-wallpapers) it's best to add a file diversion using dpkg-divert. Otherwise, the package system doesn't know that the file in that location is yours and it will get overwritten whenever the package is updated. Although ubuntu-wallpapers changes infrequently, it's likely to change when you upgrade your system to a new Ubuntu release.

Using dpkg-divert is simple:

sudo dpkg-divert --local --rename /usr/share/backgrounds/warty-final-ubuntu.png

This renames the file with a .distrib suffix, and you're now free to add your own version of the file. Package updates will affect the .distrib file and not your replacement.

I set the background I wanted using a symlink:

sudo ln -s autumn-dusk.jpg /usr/share/backgrounds/warty-final-ubuntu.png

If you want to use one of your own files, just make sure it's accessible to other users. All the parent directories need to have read and execute permissions. You can check this with:

sudo -u nobody file /usr/share/backgrounds/warty-final-ubuntu.png

If it's not accessible, you'll get a Permission denied message from file.