Download Windows 7 LogonUI Background Changer

This tool will allow you to customize the logon screen for Windows 7 without touching the registry at all. windows_seven_____7__by_youness_toulouse

Simply download the piece of software and make sure to Run as Administrator.

This tool has many features that make it easy to use:

The enable option allows you to change the logon background. The disable option restores the logon background back to default with just one click.
Resizes the selected image to your current resolution.
Finds the highest JPG compression that would still keep it under 256kb. (Logon backgrounds cannot exceed this size).
Saves the selected image as background Default.jpg and creates a backup of the original.