Hello All
As you all might be aware that Windows 7 does allows to change login screen image but by mangling with registry and some other tweaks as given below
A DWORD value named OEMBackground in the HKLM\Software\Microsoft\Windows\CurrentVersion\Authentication\LogonUI\Background key is checked which defines whether custom login screen is enabled or not and if enabled %windir%\system32\oobe\info\backgrounds\backgroundDefault.jpg is set as login screen image or depending on screen resolution like background768x1280.jpg
Also there are many related free software utilities are available which can do the job easily and quickly such as one at http://www.murgee.com/logon-controller/ which allows to do everything said above with few clicks and reverting to default screen is also available. Wanna try?
Regards