With this Tweaks Logon Screen changer you can change the windows 7 logon screen background with the image of your choice.
Download and extract the application and run it. then you will find something like this.
Now Click on change logon screen and browse the image of your choice and click change. Thats it. the logon screen changes successfully.
You can test the change by simply clicking the test button.
You can also get back to your default logon screen by simply clicking Revert to default logon screen.
Download Tweaks Logon