Extremely frustrating CSS issue!

My background image just won’t appear! Why?

Here is the actual html:

<div id="wpinfobox">
  <h2 id="wpinfobox-title"><strong>The title</strong></h2>
   <div>
     <ul>
        <li>One</li>
         <li>Two</li>
     </ul>
    </div>
 </div>

And here is the CSS:

[CSS moderated as per the Forum Rules. Please just post a link to your site.]

The background image just will not appear whatever I try! But it should. Anyone extremely clever out there??