The field value doesn’t provide the url to the file, so you would have to either serve the files directly from the content folder instead of from the media folder, or make the connection between the field value and the images in the page (by querying the images of the page), see https://kql.getkirby.com/ (then add the filename to the images query).
Must admit that I’m not really familiar with KQL. And I think there will be improvements to image handling once KQL makes it into the Kirby core.
As regards thumbs, I created a simple example route here: Permalinks for resized images - #3 by texnixe
But as stated in the post, this approach either needs authentication or at least some security handling.