WordPress custom featured image labels.

functions.php

<?php

function demo_enqueue_scripts() {
    wp_enqueue_style( 'twentysixteen', get_template_directory_uri() . '/style.css' );
    wp_enqueue_style( 'demo', get_stylesheet_directory_uri() . '/style.css', array( 'demo_enqueue_scripts' ) );
}

add_action( 'wp_enqueue_scripts', 'uitgelichteafbeelding_enqueue_scripts' );

function demo_init() {
	register_post_type( 'project', array(
		'public'   => true,
		'label'    => 'Projecten',
		'labels'   => array(
			'featured_image'        => __( 'Project afbeelding', 'demo' ),
			'set_featured_image'    => __( 'Project afbeelding instellen', 'demo' ),
			'remove_featured_image' => __( 'Project afbeelding verwijderen', 'demo' ),
			'use_featured_image'    => __( 'Gebruik project afbeelding', 'demo' ),
		),
		'supports' => array(
			'title',
			'editor',
			'thumbnail',
		),
    ) );
}

add_action( 'init', 'demo_init' );