1. Type regedit in RUN or Startmenu search box and press Enter. It'll open Registry Editor.
2. Now go to following key:
3. Now in right-side pane, change value of OEMBackground to 1
4. Now go to following folder:
Here %windir% represents "Windows" folder present in the system drive where Windows 7 is installed.
5. Create a new folder "info" inside oobe folder. Then again create a new folder "backgrounds" inside info folder. So the final path would be:
6. Now copy any desired wallpaper to this new "backgrounds" folder but following conditions must be followed:
The image name must be backgroundDefault.jpg.
The image size must be less than 256 KB.
The image format must be JPG.
7. That's it. Once you copy the wallpaper to "backgrounds" folder, your Windows 7 login screen will start using this new wallpaper as background. You can check the new login screen background by logging off or locking Windows.