In Windows 7 you can change the login screen background without using any thrid party softwares. By default custom welcome Screen background is disabled.You have enable Custom background to change the Login Screen Image in Windows 7.
How to Enable Custom Background
Enable custom login screen from Registry Editor.
Type Regedit in Search box in Start Menu and press enter.
Now Registry Editor Window would open.
Right Click on HKEY_LOCAL_MACHINE folder and go to Find to search OEMBackground.
If you find OEMBackground then its ok or else you have create one.
Right Click on HKEY_LOCAL_MACHINE folder then click on NEW option and DWORD Value.
Set the name as OEMBackground and change the value to 1.
After this go to %windir%\system32\oobe
Search for the folder named “info”.If it is not there create a new folder and name it as info.Create a new folder inside info and name its as backgrounds.Now copy any image any paste it in backgrounds folder.
Change the name of the image to backgroundDefault.jpg
(Note:Rename the image as backgroundDefault.jpg is just an extension)
No need to restart the system.Just logout or click windows lock (win+L).
Thats it you have succesfully changed Windows 7 Login Screen Background.