Having deconstructed a few sites it seems clear that many designers create their pages in Fireworks (or similar) then export gif slices to Dreamweaver tables. I'm trying to do this but am having trouble when the gifs I export are to be used as rollover images. i.e. how do I create the post roll over image and ensure that it is the same size and in the same location as the preroll image?

I use exactly the same image and edit it in fireworks, photoshop or flash and then save it in a buttons/rollovers folder as the alternate image. That way everything stays the same size.
I then use the insert rollover function in DW and browse to assign the correct image to its state.