WordPress: add custom image sizes to the list of choices in Insert Media.

wp-insert-custom-media-sizes.php

<?php
/**
 *	Custom Image Sizes
 *	- adds all custom image sizes to the list of choices in Insert Media
 */
add_filter( 'image_size_names_choose', function ( $sizes ) {
	global $_wp_additional_image_sizes;
	if ( !empty( $_wp_additional_image_sizes ) ) {
		foreach ( $_wp_additional_image_sizes as $id => $data ) {
			if ( !isset( $sizes[$id] ) )
			$sizes[$id] = ucfirst( str_replace( '-', ' ', $id ) );
		}
	}
	return $sizes;
});