config.yml

# config.yml

services:
  acme.twig.gravatar_extension:
    class: Ray\CoreBundle\Twig\GravatarExtension
      tags:
        - { name: twig.extension }

GravatarExtension.php

<?php

// Acme\DemoBundle\Twig\GravatarExtension

namespace Acme\DemoBundle\Twig;

class GravatarExtension extends \Twig_Extension
{
    private $secure_request = false;

    public function getFilters()
    {
        return array(
            'gravatar' => new \Twig_Filter_Method($this, 'gravatarFilter'),
            'sgravatar' => new \Twig_Filter_Method($this, 'secureGravatarFilter'),
        );
    }

    public function gravatarFilter($email, $size = null, $default = null)
    {
        $defaults = array(
            '404',
            'mm',
            'identicon',
            'monsterid',
            'wavatar',
            'retro',
            'blank'
        );

        $hash = md5($email);
        $url = $this->secure_request ? 'https://' : 'http://';
        $url .= 'www.gravatar.com/avatar/'.$hash;

        // Size
        if (!is_null($size)){
            $url .= "?s=$size";
        }

        // Default
        if (!is_null($default)){
            $url .= is_null($size) ? '?' : '&';
            $url .= 'd=' . (in_array($default, $defaults) ? $default : urlencode($default));
        }

        return $url;
    }

    public function secureGravatarFilter($email, $size = null, $default = null)
    {
        $this->secure_request = true;
    }

    public function getName()
    {
        return 'gravatar_extension';
    }
}

someview.html.twig

{{ email | gravatar }}                  # Will return default gravatar url
{{ email | gravatar(64) }}              # Will return gravatar url 64x64
{{ email | gravatar(200,'identicon') }} # Will return gravatar url 200x200 with default identicon
{{ email | sgravatar }}                 # Will return default secure (https) gravatar url