hehe/user/plugins/admin/themes/grav/templates/partials/userinfo-avatar.html.twig

{% if data.avatar %}
    <label><img src="{{ base_url_simple ~ '/' ~ (data.avatar|first).path }}" /></label>
{% else %}
    <label><img src="https://www.gravatar.com/avatar/{{ data.email|md5 }}?s=200" /></label>
{% endif %}