[SOLVED] how to change login screen background in XFCE LMDE
 

Had to play around a bit before finding out how to do it, so here's easy instructions in case somebody needs it (worked on Linux Mint Debian Edition as of april 2012):

The login screen is handled by the gdm3 application. There's a gdm3setup app that can change configuration, background etc but I never got it to work.

If you want to manually change background on the login screen, edit this file:

sudo gedit /usr/share/linuxmint/adjustments/gdm3/greeter.gconf-defaults

And change the line:

/desktop/gnome/background/picture_filename /home/rasta/kollektivet.png

to the path of your picture (can't remember what was the original one, this is the one I'm using). It needs to be a 1920x1200 size PNG file, or you'll end up with just a blue background.

You can also change theme etc of the login screen in this file.

you might need to change background picture path in the bottom line of this file as well, not sure:
/usr/share/linuxmint/adjustments/gdm3/greeter.gsettings

Now reboot (a logout won't be enough).

Cheers