Check for featured image on post save

check_for_thumb.php

<?php 
function check_for_thumb( $post ){
	
	global $post;	
	$post_type = get_post_type( $post->ID );
	$post_thumb = get_post_meta( $post->ID, '_thumbnail_id', true );

	if( $post_type === 'bildspel' && isset( $_GET['post'] ) && $_GET['action'] === 'edit' && empty( $post_thumb ) ) {
		echo "<div class='error'><p>Posten saknar utvald bild</p></div>";			
	}
	
  	if( !is_null( $post_thumb ) ) $thumb_meta = wp_get_attachment_metadata( $post_thumb, true );	
	if( isset($_GET['post'] ) && $_GET['action'] === 'edit' && !empty( $post_thumb ) ) {		
		$width = $thumb_meta['width'];
		$height = $thumb_meta['height'];	
		if( $width < '485' || $height < '360' ) {
			echo "<div class='error'><p>Utvald bild bör vara minst <strong>485 x 360 px</strong>. (Nuvarande bild är $width x $height px.)</p></div>";
		}			
	}
}
add_action('admin_notices', 'check_for_thumb');