_thumbnail_id not display when full url of image

i was using an old version of arras with a custom field of thumbs to display post thumbnails.

In the process of upgrading templates and with 6600 posts I dont want to change this by hand – so i changed field thumbs in wp_postmeta table to _thumbnail_id but the images are still not displaying.

If look into individual post db records – a post that has the thumb selected manually from nextgen gallery has the following format

_thumbnail_id ngg-14443
and displays

whereas my “older” fields show as

_thumbnail_id https://archiseek.com/wp-content/gallery/uk-england-derbyshire/0016.jpg
and doesnt display