custom-youtube-placeholder.php

<?php

/* 
** Remove line 1 when you're NOT using a MU plugin
** YouTube custom placeholder will replace all YouTube placeholders
*/   

function cmplz_my_custom_youtube_placeholder($new_src, $src){
	    $new_src = 'https://mydomain.com/my-incredible-placeholder.jpg';
	    return $new_src;
    }
    add_filter( 'cmplz_placeholder_youtube', 'cmplz_my_custom_youtube_placeholder', 10, 2 );