firstly Welcome aboard [righton]
I had a look at your site and it looks good and loads fast - great
now your zip file I looked at that too - the only thing that I could see that you could do is delete layer 4,9,3,13 as there is nothing I could see in them then to make it easier for yourself I would have put the links for the upper corner into their own folder
looks great I think and I don't think you would want to merge all the layers down to one incase you ever wanted to change something right?
anyway I have rezipped my changes so that you can see what I have done
I also looked to see what jpg quality and I chose 60 without loosing anything it is always different as it depends on the image
anyway I hope I have helped although I doubt it as you had done everything pretty well right well done [righton]
sfm