// Feaured image tweaks
function customposttype_image_box() {
remove_meta_box( 'postimagediv', 'case-study', 'side' );
add_meta_box('postimagediv', __('Featured image <span style="color:red">*</span>'), 'post_thumbnail_meta_box', 'case-study', 'side', 'high');
}
add_action('do_meta_boxes', 'customposttype_image_box');
function wpse_108013_mandatory_featured_image() {
if(!has_post_thumbnail()) { // here you check if there's a featured image
wp_die( 'A featured image is required' ); // there is no featured image
}
}
add_action( 'pre_post_update', 'wpse_108013_mandatory_featured_image' );
add_filter('admin_post_thumbnail_html', 'add_featured_image_text');
function add_featured_image_text($content) {
return $content .= '<small style="color:#999">Required</small>';
}