Filters the admin post thumbnail HTML markup to return.
Parameters
$contentstring- Admin post thumbnail HTML markup.
$post_idint- Post ID.
$thumbnail_idint|null- Thumbnail attachment ID, or null if there isn’t one.
Source
return apply_filters( 'admin_post_thumbnail_html', $content, $post->ID, $thumbnail_id );
Below is an example of how to use this filter to provide some additional text to admins on what size image to use for their featured images. If an image is currently in place, this text will show above it. If not, the help text will show just above the “Set featured image” link.
Doesn’t work with new Gutenberg.
There’s a open discussion going on for this.
https://github.com/WordPress/gutenberg/issues/12742