Logon WorkShop is a free portable tool for Windows 7 created by dejco at deviantART which not only lets you change your logon screen's background but every single thing that you see on your logon screen. That is, you can change the button size, accessibility button location, other button's placement, margins, user tile width & height, background overlay, text color, shadow and much more.
It comes with some sample logons created using this tool which can show you what this tool can do:
(9.1 MB)