WordPress sponsors plugin with custom featured image meta box for banners.

plugin.php

<?php

class PronamicSponsorsPlugin {
	/**
	 * Constructs and initalizes an sponsor plugin
	 *
	 * @param string $file
	 */
	public function __construct( $file ) {
		$this->file = $file;

		// Actions
		add_action( 'plugins_loaded', array( $this, 'plugins_loaded' ) );

		add_action( 'init', array( $this, 'init' ) );

		add_action( 'admin_enqueue_scripts', array( $this, 'admin_enqueue_scripts' ) );

		add_action( 'wp_ajax_pronamic_set_image', array( $this, 'wp_ajax_set_image' ) );

		// Sponsor
		$post_type = 'sponsor';

		add_action( 'add_meta_boxes_' . $post_type, array( $this, 'add_meta_boxes_sponsor' ) );
	}

	//////////////////////////////////////////////////

	/**
	 * Plugins loaded
	 */
	public function plugins_loaded() {
		load_plugin_textdomain( 'text_domain', false, dirname( plugin_basename( $this->file ) ) . '/languages' );
	}

	//////////////////////////////////////////////////

	/**
	 * Init
	 */
	public function init() {
		register_post_type( 'sponsor', array(
			'labels'             => array(
				'name'               => _x( 'Sponsors', 'post type general name', 'text_domain' ),
				'singular_name'      => _x( 'Sponsor', 'post type singular name', 'text_domain' ),
				'menu_name'          => _x( 'Sponsors', 'admin menu', 'text_domain' ),
				'name_admin_bar'     => _x( 'Sponsor', 'add new on admin bar', 'text_domain' ),
				'add_new'            => _x( 'Add New', 'sponsor', 'text_domain' ),
				'add_new_item'       => __( 'Add New Sponsor', 'text_domain' ),
				'new_item'           => __( 'New Sponsor', 'text_domain' ),
				'edit_item'          => __( 'Edit Sponsor', 'text_domain' ),
				'view_item'          => __( 'View Sponsor', 'text_domain' ),
				'all_items'          => __( 'All Sponsors', 'text_domain' ),
				'search_items'       => __( 'Search Sponsors', 'text_domain' ),
				'parent_item_colon'  => __( 'Parent Sponsors:', 'text_domain' ),
				'not_found'          => __( 'No sponsors found.', 'text_domain' ),
				'not_found_in_trash' => __( 'No sponsors found in Trash.', 'text_domain' )
			),
			'public'             => true,
			'publicly_queryable' => true,
			'show_ui'            => true,
			'show_in_menu'       => true,
			'query_var'          => true,
			'rewrite'            => array(
				'slug' => _x( 'sponsors', 'slug', 'text_domain' ),
			),
			'capability_type'    => 'post',
			'has_archive'        => true,
			'hierarchical'       => false,
			'menu_position'      => null,
			'menu_icon'          => 'dashicons-megaphone',
			'supports'           => array(
				'title',
				'editor',
				'author',
				'thumbnail',
				'excerpt',
				'comments',
			),
		) );
	}

	/**
	 * Admin enqueue scripts
	 */
	function admin_enqueue_scripts() {
		wp_enqueue_script(
			'sponsors',
			plugins_url( 'admin.js', $this->file ),
			array(),
			'1.0.0',
			true
		);
	}

	//////////////////////////////////////////////////

	/**
	 * Add meta boxes sponsor
	 */
	public function add_meta_boxes_sponsor( $post ) {
		add_meta_box(
			'sponsor_medium_rectangle_banner',
			__( 'Medium Rectangle Banner', 'text_domain' ),
			array( $this, 'meta_box_sponsor_medium_rectangle_banner' ),
			'sponsor',
			'side',
			'default'
		);

		add_meta_box(
			'sponsor_rectangle_banner',
			__( 'Rectangle Banner', 'text_domain' ),
			array( $this, 'meta_box_sponsor_rectangle_banner' ),
			'sponsor',
			'side',
			'default'
		);
	}

	/**
	 * Meta box sponsor details
	 */
	public function meta_box_sponsor_medium_rectangle_banner( $post ) {
		echo '<p>';
		echo wp_kses( sprintf(
			__( 'Medium Rectangle Banner is %s × %s.', 'text_domain' ),
			'<code>' . 300 . '</code>',
			'<code>' . 250 . '</code>'
		), array( 'code' => array() ) );
		echo '</p>';

		echo $this->post_image_html( array(
			'post_id'  => $post->ID,
			'meta_key' => '_sponsor_medium_rectangle_banner_id',
			'labels'    => array(
				'select' => __( 'Select Medium Rectangle Banner', 'text_domain' ),
				'remove' => __( 'Remove Medium Rectangle Banner', 'text_domain' ),
			),  
		) );
	}

	/**
	 * Meta box sponsor medium rectangle banner
	 */
	public function meta_box_sponsor_rectangle_banner( $post ) {
		echo '<p>';
		echo wp_kses( sprintf(
			__( 'Rectangle Banner is %s × %s.', 'text_domain' ),
			'<code>' . 180 . '</code>',
			'<code>' . 150 . '</code>'
		), array( 'code' => array() ) );
		echo '</p>';

		echo $this->post_image_html( array(
			'post_id'  => $post->ID,
			'meta_key' => '_sponsor_rectangle_banner_id',
			'labels'    => array(
				'select' => __( 'Select Rectangle Banner', 'text_domain' ),
				'remove' => __( 'Remove Rectangle Banner', 'text_domain' ),
			),  
		) );
	}

	/**
	 * WordPress AJAX set image.
	 *
	 * @see https://github.com/WordPress/WordPress/blob/4.1/wp-includes/js/media-editor.js#L638-L660
	 * @see https://github.com/WordPress/WordPress/blob/4.1/wp-admin/includes/ajax-actions.php#L1923-L1957
	 */
	function wp_ajax_set_image() {
		$post_id  = filter_input( INPUT_POST, 'post_id', FILTER_VALIDATE_INT );
		$meta_key = filter_input( INPUT_POST, 'meta_key', FILTER_SANITIZE_STRING );
		$image_id = filter_input( INPUT_POST, 'image_id', FILTER_VALIDATE_INT );
		$labels   = filter_input( INPUT_POST, 'labels', FILTER_SANITIZE_STRING, FILTER_REQUIRE_ARRAY );

		if ( empty( $image_id ) ) {
			delete_post_meta( $post_id, $meta_key );
		} else {
			update_post_meta( $post_id, $meta_key, $image_id );
		}

		$return = $this->post_image_html( array(
			'post_id'  => $post_id,
			'meta_key' => $meta_key,
			'image_id' => $image_id,
			'labels'   => $labels,
		) );

		wp_send_json_success( $return );
	}


	/**
	 * Post image HTML.
	 *
	 * @see https://github.com/WordPress/WordPress/blob/4.1/wp-admin/includes/post.php#L1267-L1309
	 */
	private function post_image_html( $args ) {
		$args = wp_parse_args( $args, array(
			'post_id'   => null,
			'meta_key'  => '_custom_thumbnail_id',
			'mime_type' => 'image',
			'labels'    => array(
				'select' => __( 'Select Image', 'text_domain' ),
				'remove' => __( 'Remove Image', 'text_domain' ),
			),   
		) );

		$post_id  = $args['post_id'];
		$meta_key = $args['meta_key'];
		$image_id = get_post_meta( $post_id, $meta_key, true );
		$labels   = $args['labels'];

		$post = get_post( $post_id );

		$set_thumbnail_link = sprintf(
			'<a class="%s" href="%s">%s</a>',
			esc_attr( 'set-post-image' ),
			esc_attr( '#' ),
			'%s'
		);

		$content = '';

		$content .= sprintf(
			'<div class="pronamic-image-control" data-image-control="%s">',
			esc_attr( wp_json_encode( $args ) )
		);

		if ( $image_id && get_post( $image_id ) ) {
			$thumbnail_html = wp_get_attachment_image( $image_id, array( 266, 266 ) );

			$content .= '<p>';
			$content .= sprintf( $set_thumbnail_link, $thumbnail_html );
			$content .= '</p>';

			$content .= '<p class="hide-if-no-js">';
			$content .= sprintf(
				'<a class="%s" href="%s">%s</a>',
				esc_attr( 'remove-post-image' ),
				esc_attr( '#' ),
				esc_html( $labels['remove'] )
			);
			$content .= '</p>';
		} else {		
			$content .= '<p class="hide-if-no-js">';
			$content .= sprintf( $set_thumbnail_link, esc_html( $labels['select'] ) );
			$content .= '</p>';
		}

		$content .= '</div>';

		return $content;
	}
}