Thumbnail of webcomic as open graph image

There’s no administrative option for this, but you can use the webcomic_opengraph filter to change any of the OpenGraph tags Webcomic adds. Assuming you set the cropped image as the Webcomic post thumbnail, you could do something like this:

function custom_webcomic_opengraph($output, $object, $collection) {
  // Only filter for individual Webcomic posts.
  if (!is_singular() || !$collection) {
    return $output;
  }

  // You can optionally specify a size, like get_the_post_thumbnail_url('large')
  $output['og:image'] = get_the_post_thumbnail_url();

  return $output;
}

add_filter('webcomic_opengraph', 'custom_webcomic_opengraph', 10, 3);

How can I set the cropped image as the Webcomic post thumbnail? The usual Featured Image box doesn’t appear as an option on the Edit Webcomic page. I am using Inkblot.