I would also love to use the default Thunderbird photo icon for when one isn't found, as its nice and subtle in the message window.
Note:
http://en.gravatar.com/site/implement/images/The 'mm' photo might do in the mean time in addition to the current three options ('identicon', 'monsterid' and 'wavatar'), but the
'404' option would provide a response that could be used for detecting if an image was not returned and thus using the default icon for those instances.
Aside from that problem this plugin is fantastic!
Edit:
In the mean time I have set the line:
user_pref("extensions.contactphotos.gravatarD", "mm");
in: C:\Users\{username}\AppData\Roaming\Thunderbird\Profiles\{userid}.default\prefs.js
but I really don't like any of Gravatar's default icons unfortunately.