Change the Windows 7 Logon Background
Warning: This post deals with editing the registry. If you don’t know what that is stop right here & ask someone else to help you. Using the registry incorrectly can cause serious problems that may require you to reinstall Windows. Use the registry at your own risk.
Pick the image you want to use & save it so that it is 250KB or less. (Paint.net is a great photo editor.) Name the file backgroundDefault.jpg. Copy it to C:\Windows\System32\oobe\info\backgrounds. Create any folders in that path that do not exist.
Open regedit & browse to HKLM\SOFTWARE \Microsoft\Windows\CurrentVersion\Authentication\LogonUI\Background. Create a REG_DWORD called OEMBackground & give it a value of 1. Close regedit.
Now lock your session & you should see the custom background. No reboot needed.
You can also use a GPO instead of editing the registry. Browse to Computer Configuration>Administative Templates>System>Logon & enable Always use custom logon background.