使用HTTP发布缩略图完整网址

I currently have a form in my wordpress backend showing a list of posts with their thumbnail.

However when I inspect the source, I see that all the URLs for the images start with 2 forward slashes before the domain.

This is a pain because I am sending this list of posts in an email via wp mail and with that URL being the way it is, it's obviously looking for a local image at the other end.

 <?php 
 if( has_post_thumbnail() ) {
 $thumb_id = get_post_thumbnail_id();
 $thumb_url = wp_get_attachment_image_src($thumb_id,'thumbnail-size', true);
 echo '<img src="' . $thumb_url[0] . '"/>';                                                     
 } else { ?>
 <img src="<?php echo get_site_url(); ?>/wp-content/plugins/scp-bookings/custom/images/hotel-512.png" width="150" height="150" >
 <?php } ?>

I need to show the full URL (removing the // before the url).