The 'talk' icon in Echo talk-page notification emails is loaded via HTTP:
<img src="http://bits.wikimedia.org/static-1.22wmf13/extensions/Echo/modules/icons/Talk.png" alt="" height="30" width="30"></td>
Because the image is loaded remotely, many mail readers (including Gmail) won't show it by default, and it'll trigger a prompt to display remote images which must be clicked on before you can see the icon.
I would recommend embedding the icons as data URIs if possible, so they don't have to be loaded remotely. (Hopefully this isn't a compat issue...)