Windows 7 Logon Editing Guide
That's a very useful link, that gives a good insight into the different things you can change on the logon screen, without installing Stardock's products. You mightn't like that idea, but using Resource Hacker is a lot easier once you get used to it. Use the registry entry in the answer above. The OEMBackground entry probably won't exist, so you'll have to create it as a 32bit D-WORD and set it to a value of 1.
Furthermore, you're going to need to create two new folders:
Go to Computer: Windows drive: Windows Directory: System32: oobe
Create folder with name: info
Create folder with name: backgrounds
Create a picture file <256KB and the same size as the resolution for your VDU (monitor).
Place picture file inside the new 'backgrounds' folder.
Leave the GUI. You don't have to restart or shutdown... you can log-off or switch user, or even just press Alt+Ctrl+Del, and it should show your new logon screen background.
Now we just need a script to randomly pick a picture out of a bunch of them...