You already know trick to change Windows 7 Logon background wallpaper and if you are not comfortable with that then you may use freeware application like Windows 7 Logno changer and windows 7 LogonUI Background Changer.
Next thing which you need while changing Windows 7 logon background is collections of cool, stunning wallpapers for using as background on your Windows 7 logon screen. If you are looking for such wallpaper then check out below logon background wallpapers.
Windows 7 Logon Background Wallpapers
All of above wallpapers are resized and compressed to meet maximum size requirement of 256Kb for Windows 7 Background wallpapers.