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I can't find any cut-and-paste code for your exact situation, but here's some javascript that lets you [url=http://www.faqts.com/knowledge_base/view.phtml/aid/4005/fid/192]access the background image of a table cell[/url]. Scripting the specific behavior you're asking for could start from there.
This may get complicated -- there are separate techniques for separate browsers. Can you give it a try? I'll also give it shot when I can clear some time for experimenting.