Ubuntu – /etc/lightdm/unity-greeter.conf file missing

Change the wallpaper to match your "dynamic" backgoroud or, change it to - something like a black, or dark purple background so you don't notice the switch, How do I change the wallpaper of the login screen? , otherwise you will notice when it changes from the default to your dynamic.

Maybe its a bug, who knows? If you turn off the dynamic wallpaper it will still show a transition from a purple background to a custom wallpaper.

Note to benefit others: Answer accepted as it improved LightDM transition, however it did not fully resolve the issue. Complete resolution would be nice, its a "paper over the cracks" solution (the black background).

unity-greeter.conf was replaced with gsettings in unity-greeter 0.2.1, and hence unity-greeter.conf has been removed. To change a global setting you run

sudo -u lightdm dbus-launch gsettings set com.canonical.unity-greeter <setting-name> <setting>

For string settings, I think you need quotes around the setting value.

For example:

sudo -u lightdm dbus-launch gsettings set com.canonical.unity-greeter background '/usr/share/backgrounds/Twilight_Frost_by_Phil_Jackson.jpg'

I use dconf-editor to browse through these settings, it's available in the dconf-tools package.

Now, what Mitch is saying does work, but it seems it might require you to change the wallpaper twice for the greeter to pick up the change. What you do is change your wallpaper, then change it back to what you want. Now when you click on different users in the login screen you will see different wallpapers. If you want the global change for users who haven't set a custom wallpaper, use the method above.