You can ofcourse use some programs that changes logon screen but these tips will help you do the same without spending any money to those kind of programs.
Press Win+R key and Type “REGEDIT” at run prompt (without quotes). Alternately, Press Start, click on Run and type.
Locate following registry key:
HKey_Local_Machine\Software\Microsoft\Windows\CurrentVersion\Authentication\LogonUI\Background
Now in right-side pane, change value of OEMBackground to 1 (originally it should be 0).
Now go to %windir%\system32\oobe
Create a new folder info and another folder backgrounds within newly created info folder
Folder path should look like this %windir%\System32\oobe\info\backgrounds
Now place your desired logon wallpaper within this background folder with file name as backgroundDefault.jpg. Here is an interesting thing, you can place multiple versions of pictures for different resolutions. Windows with place that picture according to the resolution of screen. You just have to mention the resolution in names
Images Name with resolutions
backgroundDefault.jpg
background768×1280.jpg (0.6)
background900×1440.jpg (0.625)
background960×1280.jpg (0.75)
background1024×1280.jpg (0.8)
background1280×1024.jpg (1.25)
background1024×768.jpg (1.33-)
background1280×960.jpg (1.33-)
background1600×1200.jpg (1.33-)
background1440×900.jpg (1.6)
background1920×1200.jpg (1.6)
background1280×768.jpg (1.66-)
background1360×768.jpg (1.770833-)
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.
You are all done. Restart your system to see the effect.