How do I change the Windows 7 login screen background?

It starts with registry editing

Don’t let it scare you, but we start by making a single, simple change to the Windows registry.
Fire up regedit, most easily by clicking on Start, then Run, or typing the Windows key + R, and typing regedit in the resulting dialog:

You may need to confirm a UAC prompt.
In Registry Editor, expand by clicking the small triangle to the left of each:

 HKEY_LOCAL_MACHINE
.. SOFTWARE
…. Microsoft
…… Windows
…….. CurrentVersion
………. Authentication
………… LoginUI

and then click on the Background item:

Double click on OEMBackground in the right-hand pane and change the value from 0 to 1 (if it’s already 1 you don’t need to change it):

That’s it for registry editing! Close regedit.

A special file in a special place

Next we’ll create a couple of folders (if they don’t already exist), and place our desired background image there.
Fire up Windows Explorer and navigate to C:\Windows\System32\oobe (the “oobe” stands for Out Of Box Experience):

In that folder you will need to create a new folder, “info”, if it does not already exist.
In the info folder you’ll then also need to create another new folder, “backgrounds”:

Now, place your desired background image into that folder, with the following conditions:

• It must be named backgroundDefault.jpg
• It must be smaller than 250k in size
• It probably should be the same width and height as your screen’s resolution (1600 x 900 in my example).

That’s it.
You’re done!
Log out and you’ll see your new background image as the login screen background image: