Configure lightdm (the display manager) to use a custom static background for the login page instead of the default setting of using each users wallpaper as the background for their login page. This does not affect the lock screen. This should work for Ubuntu 12.04 and up. Based on this askubuntu answer for the background and this askubuntu answer for the dot grid.

Setup a custom login background

• Open terminal and get root

    sudo -i
  

• Allow the lightdm user to create a connection to the X server

    xhost +SI:localuser:lightdm
  

• Switch user to lightdm

    su lightdm -s /bin/bash
  

• Turn off the dynamic switching of the background

    gsettings set com.canonical.unity-greeter draw-user-backgrounds 'false'
  

• Disable the dot grid on the login background

    gsettings set com.canonical.unity-greeter draw-grid false
  

• Change the default background with a picture of your choice

    gsettings set com.canonical.unity-greeter background '/foo/wallpaper.png'
  

• Prevent the lightdm user from creating a connection to the X server

    xhost -SI:localuser:lightdm
  

Revert the changes

• Open terminal and get root

    sudo -i
  

• Allow the lightdm user to create a connection to the X server

    xhost +SI:localuser:lightdm
  

• Switch user to lightdm

    su lightdm -s /bin/bash
  

• Turn on the dynamic switching of the background

    gsettings reset com.canonical.unity-greeter draw-user-backgrounds
  

• Enable the dot grid on the login background

    gsettings set com.canonical.unity-greeter draw-grid
  

• Change the default background to the default value

    gsettings reset com.canonical.unity-greeter background
  

• Prevent the lightdm user from creating a connection to the X server

    xhost -SI:localuser:lightdm