Top margin being ignored

http://www.genosfear.com

I just installed a blank template to play with and try and learn what does what with WordPress. I made a simple background, aligned it at the top center with no repeat but as soon as i did, the gap at the top of the page between the black boarder and the browser tabs disappeared.

I looked if any of the code (other than what i added for the background) had changed and nothing had.

The margin was still set at 20. I changed the value a few times to see if that affected it but still nothing.

This problem doesn’t happen if i just add a small picture with no extra code and let it auto repeat all over the page.

Any help would be awesome.

Mike.