https://www.wplokal.com/blog/membuat-plugin-wordpress-sederhana/

contoh-plugin-wplokal.php

<?php 
/*
Plugin Name:  Contoh Plugin WPLOKAL
Plugin URI:   https://www.wplokal.com/blog/membuat-plugin-wordpress-sederhana/
Description:  Tutorial membuat plugin sederhana
Version:      1.0.0
Author:       Dodi Hidayatullah
Author URI:   <a href="https://www.wplokal.com">https://www.langitstudio.com/</a>
License:      GPL2
License URI:  https://www.gnu.org/licenses/gpl-2.0.html
Text Domain:  wplokal
Domain Path:  /languages
*/

/**
* hook filter the_content()
*/
function wplokal_the_content_filter($content) {
  //ganti https://www.wplokal.com/demo/my-image.jpg dengan link image kalian
  $image = '<br><img src="https://www.wplokal.com/demo/my-image.jpg">';
  return $content.$image;
}

add_filter( 'the_content', 'wplokal_the_content_filter' );