Like you, I've messed with the alternatives directory the /etc/gdm directory etc etc.
But I stumbled on an easy way to change the login screen wallpaper.
1. cd into /usr/share/images/desktop-base
2. Rename login-background.svg to what it really should be or whatever you want... spacefun-login.svg or ugly.login-bkgrd.svg.
3. Make a new symlink to the wallpaper you want, to /usr/share/images/desktop-base/login-background.svg
e.g. ln -s /path/to/new/background /usr/share/images/desktop-base/login-background.svg
That should do it. I've checked with a few different pictures and every time I log out, it shows the new linked background.