Using http://www.perkinsblog.net/blog/index.php/2010/05/change-windows-7-lock-screen/ as a source for where this is stored in the registry and what file is used, you can schedule some batch file to copy in a new one on schedule.
This is not an offer to teach windows administration but to provide how I would solve it.
I'm currently using a program called Tweaks Logon Changer to change the background of my lockscreen on Windows 7, and it does that just fine. But I'm wondering if there is a program that can do the same thing, and cycle through multiple backgrounds instead of using just one. Thanks!