Steps to Change Windows 7 Logon Background Image
1. press win+r key and Type Regedit at run prompt
2. Locate following registry key
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
Please 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.