Use something like this. The main thing you are looking at is the position: fixed attribute. This keeps the position of the container fixed relative to the browser window itself, so since (in my html not posted) the header <div> is the first one displayed it is at the top of the page, and thus it will remain there even when scrolled. The container id below it was only created so that you can apply it to another <div> below it creating a scroll bar to see it work. So what this does is display an image (thumbs.gif) across the top of the page at all times.
background: url(thumbsup.gif) top left;