Display placeholder image when there's no thumbnail for the post

post_thumbnail_html-placeholder.php

<?php
/**
 * Filter to put a placeholder in, when there's no thumbnail set for post (which is the case for a lot of legacy posts)
 */
add_filter( 'post_thumbnail_html', function( $html, $post_id, $post_thumbnail_id, $size, $attr ) {
		// We have a thumb, return it
		if ( $html )
			return $html;

		$placeholder_url = 'http://full.url.to/placeholder_image';

		$attr = wp_parse_args( $attr, [ 'class' => '' ] );

		return sprintf( '<img src="%s" class="%s" title="%s">',
			esc_url( $placeholder_url ),
			esc_attr( $attr[ 'class' ] ),
			esc_attr( get_the_title( $post_id ) ) );
	}, 10, 5 );