The slices (gifs) for your menu have different heights, which explains the problem.
EIther recode in DW without the use of tables, and use css to format the menu - or if your html/css skills are lacking (and you do not wish to learn html/css), switch to a WYSIWYG editor like Google Web Designer, Muse, Xara Webdesigner, Freeway, Kompozr, and so on.
Thanks very much for your response. The only problem is, on older versions of DW, there was a section within the properties window to make border 0. This has now been removed and for
older people like me, it makes it so difficult to amend/append amongst the raw code. I realise that it may be outdated but I just want a quick fix. I'm not interested in SEO or old fashioned modes. I just want a simple way of putting an image on a page without resorting to find/buy/learn a new software package....