title: Setting a background image in a div, position options
body: <p>I know how to set a background in a div like:</p>
  
  <pre><code>background: url(/images/image.gif) no-repeat top right;
  </code></pre>
  
  <p>Sometimes I need more fine grained control, other than say top, center or bottom.</p>
  
  <p>I have seen people using 'em' in the position section, what is that doing?</p>