Get post attached image to display in admin dashboard

You need the attachment ID – which is the same as the “thumbnail ID” – not the post ID. It’s confusing. Maybe by WP 5.0 they’ll update the nomenclature!

So, I think this’ll work:

function attached_image_content( $column_name, $post_ID ) {
    if ( $column_name == 'foobar' ) {

        //Change the image size from default 'thumbnail' here
        $post_attached_image = wp_get_attachment_image_src( get_post_thumbnail_id( $post_id ), 'medium' );
        if ( $post_attached_image ) {
            //as you probably know, the url is the first array value
            echo '<img style="width:100%;" src="' . $post_attached_image[0] . '" />';
        }
    }
}
add_action( 'manage_cpt_posts_custom_column', 'attached_image_content', 10, 2 );