It's because you have your image buttons inside ul and li tags and unfortunately browsers give ul tags different top and side (indents) margins.
Luckily the cure was simple, add margin-top: 0; in the stylesheet:-
/* ----------------------------- start #nav ----------------------------- */
#nav { margin-top: 0; }
#nav img {...............
Code downloaded to my PC will be deleted in due course.
WIN7; IE9, Firefox, Opera, Chrome and Safari for Windows; screen resolution usually 1366*768.
Also IE6 on W98 with 800*600 and IE8 on Vista 1440*900.