WordPress: Add help text to the featured image metabox.

wp-functions-featured-image-help.php

<?php
/**
 * Add guidance text to the featured image metabox.
 * Filter: admin_post_thumbnail_html
 *
 * @param  string   $content  Meta box HTML
 * @param  integer  $post_id  ID of post
 * @return string
 */
function cmfy_featured_image_help_text( $content, $post_id ) {
	// Limit text to certain post types
	if ( 'page' == get_post_type( $post_id ) ) {
		$content .= sprintf( '<p>%s<p>', __( 'For best results, this image should be 900 pixels wide x 506 pixels tall or else it could be cropped unexpectedly. ', 'cmfy' ) );
	}
	return $content;
}
add_filter( 'admin_post_thumbnail_html', 'cmfy_featured_image_help_text', 10, 2 );