add_filter( 'admin_post_thumbnail_html', 'twk_add_featured_image_instruction');
function twk_add_featured_image_instruction( $content ) {
	return $content .= '<p>The bigger the image the better. They will be centrally cropped with a landscape aspect ratio of 2:1.</br> Image checklist:
<ul><li>landscape</li>
<li>at least 1600px wide</li>
<li>twice as wide as they are high</li>
</ul>
E.G. 1600px wide x 800px high</p>
<p>Avoid images with details such as faces near the edges. The focal point of images should be towards the centre rather than towards the edges</p>';
}