I use many gif images in the adverts on my UBB pages.
They appear to be animated as they contain several different frames, stored and sequentially reproduced.
To all intents and purposes*, they may be used in the same fashion as jpgs.
Be careful though, your clients will take liberties with the banner size and weight(kb). Write a spec. sheet that gives guidance with regards to these issues.
* I'm talking specifically with regard to displaying as an advert on your pages.
