Whatever the problem, no matter how big or how small it can be solved using tables, well I think at least this problem can be.
What I would do is have 2 separate <tr>'s, one for the top, and one for the bottom, inside the <tr> tag that your ad is in write background="whtever_your_pic_is.gif" and that will confine the background to that specific cell.
tip: tables can get really complicated so if I were you I would create a new web-page and then build up you table gradually from scratch, that way if there are any error's you'll be able to spot them straight away, when your done just add your content. Also set it out neatly in blocks of text with indents in ect., so that if you ever need to modify it you can do so easily.
Sorry if I'm being patronising or haven't even answered you question.