Quick, sloppy, and gratefully cribbed from rfurgy’s comment on a post about hacking the new GDM, which incidentally his comment was the only method that worked out of all the various ways being discussed. Maybe it’s me, maybe it’s not but this method, while kind of silly, actually does make a change though ymmv.

The fugly wallpaper is kept in /usr/share/images/xsplash as bg_[screen resolution].jpg along with the logos and something called a throbber. Make backups of these and tuck them away in a safe place in case you want to revert back.

1. Make backups of all the files in /usr/share/images/xsplash just because it is the responsible thing to do
2. Find an image that you want to use for your login background
3. Crop it to the resolution of your screen
4. Generate resized versions for the six sizes contained in the folder
5. Copy your new images into /usr/share/images/xsplash
6. If you are using a light or white colored back ground you might want to invert the color in the logo images
7. Log out or reboot
8. Enjoy a little less ugly in your life

I’d take a screenshot but gdmflexserver –xnest tells me it hasn’t been implemented and I’m too lazy to look for another way. Shaky and dimly lit iPhone pictures are worth a handful of words.