Adding an header image?

[azure_chaos]

Hi,

I'm the maintainer of a comm primeval_fanfic and we've been trying to add a header image. We were previously using Flexible Squares and when we couldn't get it to add the header we thought maybe the theme was the problem and so tried swapping to components, which I'm using in my own lj. We've tried every tutorial that we can get our hands on and currently the code that we're trying to use is below. It's a plus account but there's no ads showing up for me and I wondered if the two issues were some how connected.

body {
margin-top: 317px !important;
background-image: url(oururl) !important;
background-repeat: no-repeat !important;
background-position: top center !important;
margin-left: auto;
margin-right: auto;
}

Any help would be greatly appreciated.
Thanks in advance,
Nate