Turning On Thumbnail Support for Custom Post Types

If you read through the functions.php file in any theme, you'll see a few lines of code that look like this:

add_theme_support('post-thumbnails');

These features are for WordPress's 2 default types: Posts and Pages. If you are working with custom post types, you'll have to add this line of code to your (for example) my-custom-post-types.php file located in your mu-plugins folder. You'll add thumbnail support inside the register_post_type() function, as shown below:

function my_custom_post_types() {
  register_post_type('event', array(
    'supports' => array('title', 'editor', 'thumbnail'),
  ));
}

Now, when your content editors are looking at the custom post, for example, "Event", they will see the "Featured image" option in the WP Editor (lower-right hand side).