Last week my backdrop cycler trial expired and I was stuck with a single backdrop so I wrote my own. Ok, it's not as feature rich as any of the others available on trial licences but it'll never run out.
It only does bitmaps (.bmp's) and it doesn't save your settings - yet.
I'd also like it to minimise to the system tray instead of just on top of the start button but what's there isn't bad for an evening's messing around!
Download it, install it, break it and complain about the bugs
(Includes all system updates for Win98+)