add_filter( 'get_avatar' , 'my_custom_avatar' , 1 , 5 );
function my_custom_avatar( $avatar, $id_or_email, $size, $default, $alt ) {
    $user = false;
    if ( is_numeric( $id_or_email ) ) {
        $id = (int) $id_or_email;
        $user = get_user_by( 'id' , $id );
    } elseif ( is_object( $id_or_email ) ) {
        if ( ! empty( $id_or_email->user_id ) ) {
            $id = (int) $id_or_email->user_id;
            $user = get_user_by( 'id' , $id );
        }
    } else {
        $user = get_user_by( 'email', $id_or_email );
    }

    if ( $user && is_object( $user ) ) {
        $value = rwmb_meta( 'profile_picture', array( 'object_type' => 'user' ), $user->data->ID );
        if ( $value ) {
            $avatar = "<img src='" . $value['url'] . "' class='avatar avatar-" . $size . " photo' alt='" . $alt . "' height='" . $size . "' width='" . $size . "' />";
        }
    }
    return $avatar;
}
?>
<?php