How to create a simple plugin which show/hide an html code in wordpress?

There is many ways to do it, create a shortcode to place anywhere you want or filter the post content and add what you want.

If you plugin is not activate, this will not display,

<?php
/**
 * Plugin Name: Europhonica Image/Vidéo switch (by Jules)
 */

 // the_content method
 add_filter( 'the_content', 'image_my_switch_function_content' );
 function image_my_switch_function_content($content){
      if(is_user_logged_in()){ // change with your conditions
            $content="<p>Image</p>". $content
      }
      return $content;
 }
 // shortcode method
 add_shortcode('show_image', 'image_my_switch_function_shortcode');

function image_my_switch_function_shortcode($atts, $content="null") {
     if(is_user_logged_in()){ // change with your conditions
          return '<p>Image</p>';
     }

}
// Enable the use of shortcodes in text widgets.
add_filter( 'widget_text', 'do_shortcode' );

?>

The the_content filter will be embed in any post, page… depending on your conditions.
With add_shortcode method the shortcode can be add manually (in the content or widget) or directly in the template with do_shortcode().

Directly in your template file :

 echo do_shortcode('[show_image]');

You can read more about these functions add_shortcode(), do_shortcode(), the_content

Hope it helps!