Someone just asked me how to do this... and damn is my way of doing it a pain in the butt!

Right Click on your desktop
select properties
click on the desktop tab
click "customize desktop"
click "web"
click "new" and add url of .jpg, click "okay"
back at the "web" tab, highlight your new additon and click "properties"
hit the "schedule" tab
click "edit"
hit the "schedule" tab
click the "advanced" button
enter today as the start date
check "repeat task" and set your duration.
then click "okay" about ten times to get back to your desktop.