Changing theme backgroud image.

Good day,

I’m having problem with changing my themes background image. I have a prepared image for background, but when I put in css code, it expands and displays incorrectly.
Here are the good sample of what I’m seeking for http://www.gamew0rld.com/
The background remains in place while the site is scroling down.
I’m looking forward for any help. Thank You.