If you use Debian 6 Squeeze and you are tired of looking at the childish graphical GDM login screen that is used by default, here's an easy way to change it.
If you've already tried to change it you've probably discovered that none of the old methods work. That is because Debian 6 is no longer using GDM2 but is now using GDM3.
First you need an image that is 1024 x 768. Load this image into inkscape. We need to use inkscape here because the image must be saved as type image-name.svg (scalable vector graphics). Once you have the image loaded into inkscape save it as login-background.svg. Exit inkscape. Now open up a root terminal and launch nautilus. We need to do this from a root terminal because we want nautilus to have root priviledges. Using nautilus, navigate to the place where you saved the login-background.svg image and copy it. Now navigate to the folder user/share/images/desktop-base and save the image there. You will be asked if you want to overwrite/replace the existing image – say yes. Reboot the computer and your GDM login screen should now be the image you created.
If you want to try another method you can open a root terminal and navigate to etc/gdm3. In there you will find a file called greeter.gconf.defaults. Open this file with your favourite editor (I use leafpad instead of nano because nano doesn't support the mouse). You will need to edit the line containing this