Problem customizing icons in Subtle using dd class

I’m trying to add the code

<dd class="icon email">

to my single post page (for example, http://margaretandzach.com/2007/05/10/13/) to get an e-mail icon to appear before “E-mail” in the subscribe box.

The icon does exist (at http://margaretandzach.com/wp-content/themes/gluedideas_subtle/assets/images/icon_email.gif, but I can’t get it to appear. The other icons (RSS, Digg, Del.icio.us, etc) are in the same folder and do apear using the code similar to what I posted.

Any ideas how I can get the e-mail icon to appear? Thanks!