"You can get as simple—like my "every 20 minutes" scheme
at left—or obtuse as you want with this handy tool. Want your
background to change at 9AM, noon, and 4PM, Monday through Friday?
Totally do-able. Hit the "Edit" buttons next to each field and
check out the options. I like to use mine as a peripheral gauge of
time. Since I don't let maximized windows cover my bottom AWN dock,
I can see the patterns change, and realize I've just blown 15
minutes on Flickr—again."