I have recently moved my last remaining Windows workstation to Windows 7.  The login screen now has a background image that I am not particularly fond of.  It turns out that this relatively trivial to change.

First, click on the Windows buttons (where the old start bar used to be), in the "Search programs and files" box, type regedit.  You should see regedit.exe displayed.  Right click this and select "Run as Administrator (you don't actually use Windows as an administrative user I hope).  Once you type in your admin credentials you see the old trusty registry editor interface.  Find the following registry key:

HKLM\Software\Microsoft\Windows\CurrentVersion\Authentication\LogonUI\Background