This setting is intended for original equipment manufacturers (OEMs) to customize their systems, but there’s nothing stopping you from using it yourself. All you have to do is change a single registry value and put an image file in the correct location.
When you login to your Windows 7 computer, you’ll generally see a login window with a background looking something like this.
It’s fine; there’s nothing wrong with it. But sometimes a change is good so in this article we’ll show you how to change the background image behind the login screen to anything you want.
The first step is to open up the Start Menu by clicking the orb in the lower left corner of the screen.
Now, right-click on the HKEY_LOCAL_MACHINE folder, and choose the Find option.
The search window will appear so perform a search for OEMBackground.
It will eventually be found under Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Authentication\LogonUI\Background (you could navigate there directly, but the search function is probably quicker than typing all that into the path bar.
Note: it’s possible that OEMBackground doesn’t exist on your system; if this is the case, adding a new DWORD value with the name of OEMBackground will fix things.
Once you can see the OEMBackground entry, double click to reveal its properties, and change the value from 0 (the default) to 1 (which will allow us – or the computer’s OEM as intended – to change the login screen background).
Once this is finished you can close the Registry Editor and open up Windows Explorer. We want to go to %windir%\system32\oobe, so enter that into the path bar.
Now simply select a favorite wallpaper. It should be in JPG format and less than 245 KB in size (and ideally the exact dimensions of your display to avoid stretching). Name this image backgroundDefault.jpg and place it in the backgrounds folder.
Congratulations! This isn’t the simplest process, and isn’t as simple as using a program like Windows 7 Logon Background Changer or Tweak.com's Logon Changer. but if you want to get into the particulars regarding what programs like that are doing, this is a good way to go, if a bit more complex.