title: how do I add an expiration date to an img tag?
body: <p>I'm using the Page Speed Firebug extension to help improve page performance. I have an image-heavy page, and one of the suggestions it made is this:</p>
  <blockquote>
  <p>Leverage browser caching
  The following cacheable resources have a short freshness lifetime. Specify an expiration at least one week in the future for the following resources:</p>
  <ul>
  <li><a href="http://www.mysite.com/components/com_arrcard/assets/merchant-logos/aap25.jpg" rel="nofollow noreferrer">http://www.mysite.com/components/com_arrcard/assets/merchant-logos/aap25.jpg</a> (expiration not specified)</li>
  </ul>
  </blockquote>
  <p>I know you can set Expires or Cache-Control headers on an entire page, but how do I add an expiration to a specific element? Is it even possible, or am I misinterpreting what Page Speed is suggesting?</p>