Making custom url field automatically pull in the site thumbnail

I would like to be able to add a site url in a custom field of a directory listing and automatically have a thumbnail of the site appear in posts as the featured image/thumbnail.

I think there must be a way of doing this using a plugin like STW thumbnails which uses the tag [thumb][/thumb] for adding thumbnails in posts. However, I can't work out how to go about making it work in a custom field. How also would I go about making the tag automatically surround the url so I don't need to enter it manually each time?

Would grateful for any suggestions. I have tried echo do_shortcode(‘[thumb]‘); in single.php but that doesn't work (maybe this type of tag isn't a shortcode?)

Thanks in advance.