The gravatar filter can be applied to an email address to get the url of a Gravatar image. There are three optional parameters: The size in pixels (defaults to 80), the url of a default image to use if the email is not associated with a Gravatar, and the maximum rating you want to display (defaults to 'g').

<img src="{{|gravatar(100,hifi.urls.images~'gravatar-default.jpg','r') }}" alt="Gravatar">