<?php
add_filter( 'rwmb_meta_boxes', 'prefix_attached_images_meta_box' );
function prefix_attached_images_meta_box( $meta_boxes ) {
    $meta_boxes[] = array(
        'title'  => esc_html__( 'Attached images', 'textdomain' ),
        'fields' => array(
            array(
                'type'     => 'custom_html',
                'callback' => 'prefix_get_images',
            ),
        ),
    );
    return $meta_boxes;
}

function prefix_get_images() {
    $post_id = false;
    if ( isset( $_GET['post'] ) ) {
        $post_id = intval( $_GET['post'] );
    } elseif ( isset( $_POST['post_ID'] ) ) {
        $post_id = intval( $_POST['post_ID'] );
    }
    if ( ! $post_id ) {
        return '';
    }

    $attachments = get_children( array(
        'post_parent'    => $post_id,
        'post_type'      => 'attachment',
        'post_mime_type' => 'image',
        'numberposts'    => - 1,
        'post_status'    => 'any',
    ) );
    if ( empty( $attachments ) ) {
        return esc_html__( 'No images attached', 'textdomain' );
    }
    $output = '';
    foreach ( $attachments as $attachment ) {
        $output .= wp_get_attachment_image( $attachment->ID, 'thumbnail' );
    }
    return $output;
}