docusaurus/v2/lib/theme/NotFound.js

import React from 'react';
import Layout from '@theme/Layout';

export default class NotFound extends React.Component {
  render() {
    return (
      <Layout>
        <div>404 Page Not Found</div>
        <div>
          <img
            alt="Not found"
            src="https://d2gg9evh47fn9z.cloudfront.net/800px_COLOURBOX3889253.jpg"
          />
        </div>
      </Layout>
    );
  }
}