Now with all resolutions supported!
Thanks to [link]
for making the original available for me to alter.
Some programs, like LogonStudio or LogonVista, can change the login screen for you. If you don't have one of these, here's how to do it manually (Windows 7 Only):
Launch the Registry by holding Windows and R.
Type regedit.exe into the box.
Make your way to HKEY_LOCAL_MACHINE\
In here you will find a key called OEMBackground.
(If there isn't one, make it yourself.) That key is set to 0. Change it to 1.
Close the Registry.
Pick your image.
It has to be a JPG;
it has to be the exact same resolution as your monitor;
and it also has to be less than 256kb.
(I've done all the known display resolutions here except for 2560x1440 and 2560x1600. Both impossible, as the source material was only 1920x1200.)
Go to the ‘C\
If there isn’t such a folder, make it yourself.
Save your image in there, as backgroundDefault.jpg.
Next time you log out, there it is!
(Sometimes it forgets at first; you may have to repeat the Registry procedure again.)