title: Background-position +/- X pixels
body: <p>How do I offset the <code>background-position</code>, let's say 20px, from its original position?</p>
  
  <p><strong>Like:</strong></p>
  
  <pre><code>$(".selector").animate("slow").css("backgroundPosition", "center -=20px");
  </code></pre>
  
  <p>(That obviously doesn't work... But how do I do this??)</p>