How can I change my background picture

I'm getting realy frustrated. I've been trying to change the background picture to my forum and it will not change. It stays the same as what the original background was. I typed the URL into where the URL goes but it won't change the background picture. When I try to press upload picture it says that I am not logged in or something, but this doesn't make sense because I'm definitely logged in. Any suggestions?

Thanks.

Heres my forum http://starcraft.nforum.biz/

and heres the picture i want as a background http://www.wallpaperbase.com/wallpapers/3d/3dspace/3d_space_32.jpg

when I press upload picture and copy the URL o the picture to where it says "URL" then I press enter and it say that I'm not logged in or something

It's weird because I can change my banner/logo but not my background

If you want to change the forum background.
Add this code to your CSS Stylesheet.

Code:

body {
background-image : url(IMAGE URL HERE);
}