If you are kind of person who like to customize windows 7 a lot then you gonna love Windows 7 Login changer which rotates images displayed when you get on the login page. But if you have Auto Login or have not created password for your account, you would not be able to see the change.
It allows you to select 10 images which can be used to change everytime you come to login screen. Since this is not an offical app given by microsoft make sure you make a restore point before installing it. You can download the Windows 7 login image changer and rotater from this thread of neowin forum.