Yes that’s possible I know of a few users who have such an ad.
If that ad is provided to you by a ad server you can usually add it’s code to the footer of your theme, perhaps a text widget in or near the footer.
If this code is Javascript you can not track the clicks in AdRotate but your ad server will provided stats.
If you make this code yourself you need to add it somewhere in your theme, can be the footer, but you have to make a piece of code to set the advert as your background.
Of course such an ‘self-made’ ad can be tracked by AdRotate.
