I had a look and now I see why it was fairly easy for you.
You have all of your images as seperate files. If you combine them all into a single image file the menu will load a lot quicker. It does make coding it a pain though as you have to shift the background image position around to show the various chunks.
Using the transparent GIF is a nice touch, saves having to have two different graphics for the hover and non-hovers. I assumed the hovers were an entirely different image (or part thereof). I’ll have to remember that in case I find a use for it somewhere else as it could save both time (creating the images) and possibly download size too (depends if two PNGs are larger than one GIF or not).