The Tweaks.com Logon Changer for Windows 7 is a freeware, portable tool that lets you customize the logon screen background with just a few clicks. It will give you the option to automatically resize and recompress the image to the highest quality possible. The original image is left untouched.
The application will prompt for the location of the new background image and then install the new screen behind the scenes. Tweaks.com Logon Changer will even provide a preview of your new background on a logon screen. When you have settled on a good logon background, click the Test button to see it in action.
If you want the default Windows 7 logon screen back just click Revert to Default Logon Screen and the application will automatically uninstall any customized screens.