This tool makes it really simple to change your logon screen in windows 7 to any picture you want.
Automatic Image Resize - Don't worry about the 245KB file size limit imposed by Microsoft. The Tweaks.com Logon Changer will give you the option to automatically resize and recompress the image to the highest quality possible. The original image is left untouched.
Automatic Image Backup - All OEM backgrounds and any previously used backgrounds are automatically backed up when a logon background is changed.
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.