From Arturs Sits:
With InstantLogonChanger always have your Windows 7 logon background the same as your Desktop wallpaper or set it to any other image or color without any security prompts or changing system files.
JPEG, PNG, BMP, GIF, or TIFF image formats are supported. You can fill, fit, stretch, tile, or center your image and change background color for images that are too small to fit the screen.
Made with a business user in mind, InstantLogonChanger allows you to disable all of it's notifications, such as update alerts, by enabling Hide InstantLogonChanger tray icon setting.
InstantLogonChanger also automatically adjusts logon screen text and button color so they're better visible no matter what image is used as a logon background.