Unity greeter: Changing login image..
For those who can't seem to successfully change their login image to a more personal, custom one.
The following worked for me...
Change background picture to something else (just to be sure), from your 'pictures' folder.
sudo apt-get purge unity-greeter && sudo-apt-get install ubuntu-desktop
I think the secret is also to use pics that are cropped to fit your puters screen size. IE: 16:9; 16:10; 4:3, etc...
If the pic size is too different from the screen resolution, it simply won't show up on your login screen, but will still as a background. The login screen will just show the default warty login screen.
Windows free since '09 - never looked back!