To change the background image go to Start> search for “regedit” (with out “”)> and click on “regedit.exe”
Inside Regedit follow the path bellow:
then right click on “OEMBackground” and select modify and make sure that “Value Data:” is 1. If not change it to that and hit Ok.
(if OEMBackground is not there then right click in the open space below “Default” and go to New>DWORD (32bit) Value”. Name it OEMBackground and give it a value of 1 and hit ok.)
Now go to:
(or C: can be replaced with whatever drive letter has Windows on it but generally that will be C:)
Copy and paste the .jpeg image you want into that folder and rename it “BackgroundDefault.jpg”
Now your done, next time you log out/log in you will have a new background image.
Note: Depending on who installed your Windows7 copy, so of these steps might not need to be done, but it is best to just do them all just incase. (Like for my Lenovo laptop, they had already changed the default background to a custom Lenovo background so all I had to do was switch out the image.)
Reference and thanks to: Varun Kashyap for his post at http://www.makeuseof.com/tag/how-to-change-windows-7-logon-screen/