Here is an excellent tool for Windows 7 users which allows you to choose up to 10 images and displays them randomly at each login.
You can select the images using "Choose Image" button.
Once you select all images, click on "Enable Settings and Exit" button. It'll ask you to select the initial image which will be used as the first login screen background.
That's it. Now you can enjoy different login screens at each login.
If you want to restore the default login screen, run the "Remove files and settings.exe" file included in the pack and it'll restore the default login screen.
You can download it using following link:
NOTE: You can also check other free tools to customize Windows 7 login screen at following link: