get_avatar( $current_user->user_email, 128, null, null, array('class' => array('img-responsive', 'img-rounded') ) );

$args = array(
   'class' => 'img-responsive img-rounded'
 );
get_avatar( $current_user->user_email, 128, null, null, $args );

$args = array(
 'class' => array( 'img-responsive', 'img-rounded');
);

get_avatar( $id_or_email = get_the_author_meta( 'user_email' ), $size = '60', $default, $alt, $args = array( 'class' => array( 'd-block', 'mx-auto' ) ) );

get_avatar( get_the_author_meta( 'user_email' ), '60', $default, $alt, array( 'class' => array( 'd-block', 'mx-auto' ) ) );