How do I set an image as a cell background so that it automatically duplicates? This is the typical way to create a horizontal menu bar that stretches infinite as the browser enlarges.
If is not possible with tables, are there any other ways?

as far as i know this is probably done with some sort of css, maybe using tables, i don't quite actually know. I think this is a form of "liquid design" as it has so been named.

Click her for a recent thread on liquid design

There is some discussion of tiled images here.