For the sake of simplicity, I am adding a React component. This React component is going to simulate a real life scenario of the component that is responsible for including the <header-image> web component. Let this React component be MainBody; its definition would look something like this:

import React, { Component } from 'react';

export default class MainBody extends Component {
  render() {
    return (
      <div>
        <p>This is the main body</p>
      </div>
    );
  }
}

As you can see, it shows just one line of text and nothing else. If you have a more complex component, the steps will be the same. As for the starter app, we will include this MainBody component in our App component first, which is shown here:

import React from 'react';
import logo from './logo.svg';
import './App.css';
import MainBody from './main-body/main-body.js';

function App() {
  return (
    <div className="App">
      <header className="App-header">
        <img src={logo} className="App-logo" alt="logo" />
        <p>
          Edit <code>src/App.js</code> and save to reload.
        </p>
        <a
          className="App-link"
          href="https://reactjs.org"
          target="_blank"
          rel="noopener noreferrer"
        >
          Learn React
        </a>
        <MainBody />
      </header>
    </div>
  );
}

export default App;

Here, we are simply importing the MainBody component and using it directly in the App component.