Doesn't verify that the given e-mail address has a Gravatar record.
Intuitively you'd expect this to work, but it doesn't:
[% USE Gravatar %]
[% avatar = Gravatar(email => 'foo@example.com') %]
[% IF avatar %]
<br><img src="[% avatar | html %]&s=150" alt="Gravatar image" height="150" width="150">
[% END %]