I recently came across a free automated wallpaper changer. It can be configured to show pics from different folders on your harddisk and it can also be set to download new pics from the net.
You can choose between lots of different catergories of pictures for it to download.
Most important - There are no banners or spam.....
Check it out:
http://www.multiwallpaper.com
I downloaded the full version and it works fine. Didn't have to crack it or anything.