Override get_avatar_url()

Sorry for the delay. What theme are you using?

The plugin is not replacing the url in that function.
Here is a quick fix for it.

    function profile_img_url($url, $id_or_email, $args = array()) {
        //Get user data
        if (is_numeric($id_or_email)) {
            $user = get_user_by('id', (int) $id_or_email);
        } elseif (is_object($id_or_email)) {
            $comment = $id_or_email;
            if (empty($comment->user_id)) {
                $user = get_user_by('id', $comment->user_id);
            } else {
                $user = get_user_by('email', $comment->comment_author_email);
            }
            if (!$user)
                return $url;
        } elseif (is_string($id_or_email)) {
            $user = get_user_by('email', $id_or_email);
        } else {
            return $url;
        }
        if (!$user)
            return $url;
        $user_id = $user->ID;
        $profile_post_id = absint(get_user_option('metronet_post_id', $user_id));

        if (0 === $profile_post_id || 'mt_pp' !== get_post_type($profile_post_id)) {
            return $url;
        }
        $defaults = array(
            'size' => 'thumbnail',
            'attr' => '',
        );
        $args = wp_parse_args($args, $defaults);
        extract($args); //todo - get rid of evil extract
        //Implode Classes if set and array - dev note: edge case
        if (is_array($attr) && isset($attr['class'])) {
            if (is_array($attr['class'])) {
                $attr['class'] = implode(' ', $attr['class']);
            }
        }

        $post_thumbnail_id = get_post_thumbnail_id($profile_post_id, $size, false, $attr);
        $profile_img_url = wp_get_attachment_image_src($post_thumbnail_id);
        return $profile_img_url[0];
    }
    add_filter('get_avatar_url', 'profile_img_url', 10, 3);