Without the page code I can't give a precise answer, but you need to find the parent divs or ul tag for the menu and for the image popup.
Give the menu parent div or ul tag z-index: -1; position: relative; and the parent div of the image popup z-index: 10; position: relative;
You can use position: absolute instead of position: relative if you need to, but that may affect the position in the screen. z-index needs either position: relative or position: absolute to work and it tells the browser which shows on top.
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.