/* Convert Category Image IDs to URL */
function tc_register_category_images_field() {
   register_rest_field(
    'listing',
    'category-image',
    array(
        'get_callback'    => 'tc_get_category_images_urls',
        'update_callback' => null,
        'schema'          => null,
    )
);
}

add_action( 'rest_api_init', 'tc_register_category_images_field' );

function tc_get_category_images_urls( $object, $field_name, $request ) {
$medium = wp_get_attachment_image_src( get_post_thumbnail_id( $object->id ), 'medium' );
$medium_url = $medium['0'];

return array(
    'medium' => $medium_url,
);
}