I added the ability to add featured images and it's working, but I feel like it's not using the same format as you were. Plus, the module doesn't remember the image I selected so it's not ideal. Not sure what's going on. Maybe you can adapt it.

'post_featuredImage'     => array(
    'type'          => 'photo',
    'label'         => __('Featured Image', 'bb-toolbox'),
 ),

In the save_toolbox_settings function I added:

$featuredImage = $settings['post_featuredImage'];
set_post_thumbnail( $post_id, $featuredImage );