If you need to modify the post type labels, note the labels parameter for this filter is an object, not an array.

In the example below, the post type is “employee” and the new label reference is “headshot”.

/**
 * Changes strings referencing Featured Images for a post type
 * 
 * @param 		object 		$labels 		Current post type labels
 * @return 		object 					Modified post type labels
 */
function change_featured_image_labels( $labels ) {

	$labels->featured_image 	= 'Headshot';
	$labels->set_featured_image 	= 'Set headshot';
	$labels->remove_featured_image 	= 'Remove headshot';
	$labels->use_featured_image 	= 'Use as headshot';

	return $labels;

} // change_featured_image_labels()

add_filter( 'post_type_labels_employee', 'change_featured_image_labels', 10, 1 );