Could you link to the layout, or your blog? It may help getting started and give a more complete answer.
Typically you can assign a background image to either the html or body element using css… an example would be…
body {
background: #FFF url(./images/bg_image.jpg) top center repeat-x;
}
here’s a quick tutorial on backgrounds in css which will explain what each part means and get you started. If you have questions a lot of folks here can help.
There’s some similar questions/answers to the right ====> that may help as well…