Sometimes you need to change the message.
/** Add Featured Image Text above thumbnail **/ function filter_featured_image_admin_text( $content, $post_id, $thumbnail_id ){ $help_text = '<p>' . __( '<i>Please use an image that is 800 pixels wide (min).</i>', 'ClassicPress' ) . '</p>'; return $help_text . $content; } add_filter( 'admin_post_thumbnail_html', 'filter_featured_image_admin_text', 10, 3 );
Reference Links
https://developer.wordpress.org/reference/hooks/admin_post_thumbnail_html/