Found a per user setting : launch dconf-editor then untick com.canonical.unity-greeter draw-user-backgrounds.

For a system wide setting the way is the same as for disabling greeter to use wallpapers and setting a common one from /usr/share/backgrounds:

Set a background with Ubuntu-tweak or

sudo -i

su lightdm -s /bin/bash

gsettings set com.canonical.unity-greeter background '/usr/share/backgrounds/MyTrusty.jpg'

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