The wallpaper presented to you at the logon screen in Windows 7 gets pretty boring quickly. Easy to change? Relatively. You can find some third party tools, or have a go yourself with the instructions below:
1. Press win+r key and Type Regedit at run prompt
2. Locate following registry key
HK Local Machine\Software\Microsoft\Windows\CurrentVersion\Authentication\LogonUI\Background
3. Now in right-side pane, change value of OEMBackground to 1.
4. Now place your custom image in %windir%\system32\oobe
5. Create a new folder info and background within newly created info folder
Folder path should look like this %windir%\System32\oobe\info\backgrounds
6. Now place your desired logon wallpaper within this background folder with file
name as backgroundDefault.jpg
Images Name with resolutions
NOTE: Images must be less than 256kb in size
The backgroundDefault.jpg image is loaded and stretched-to-fit when a resolution/ratio specific background cannot be found.