CSS background center problem

I’m fairly new to css and am having a perplexing problem.

when I add a line of code:

background-position: center;

it drops my backgroud image down 15-20 pixels. I dontunderstand why.

the home page is exactly how I want it and has a different style sheet from the inside pages.

http://www.alchemyhairsalon.com/2008/menu.html - for instance.
line 17 or #container is where I’m having my problem.

can someone please tell me why this would drop the background down?
everything would line up correctly if this background didn’t drop down.