I suggest that you install [FPW Category Thumbnails][1] plugin. It will cover every scenario mentioned in your question. It will save you from maintaining code in `functions.php`, and will do it visually! The built-in **FPW Post Thumbnails** will add very powerful styling to your featured images (borders, shadows, margins, backgrounds, etc). You will be able to switch themes without losing its functionality!

Your code (if you want to use it) needs 2 changes to work. Replace:

    if(!isset($_POST['_thumbnail_id'])) {

with

    if(-1 == $_POST['_thumbnail_id']) {

and replace:

    $cats = $post_categories->term_id;

with

    $cats = $post_categories[0]->term_id;


  [1]: https://wordpress.org/plugins/fpw-category-thumbnails/