Retina-sized gravatar for header of Independent Publisher theme

functions.php

<?php

function independent_publisher_retina_header_gravatar($args) {
    if(isset($args['default-image'])) {
        $args['default-image'] = str_replace('?s=100', '?s=200', $args['default-image']);
    }
    return $args;
}

add_filter('independent_publisher_custom_header_args', 'independent_publisher_retina_header_gravatar');