How to add/modify collection entity fields in apigility + doctrine + ZF3 MVC?

I have “image” table. I use apigility to get collection of items from image table. I use REST GET call for this. I have this in my ItemMapper.php file:

public function fetchAll($params)
    $query = $this->itemManager->getItems($params);
    $adapter = new DoctrineOrmAdapter(new ORMPaginator($query, false));
    $paginator = new Paginator($adapter);
    $items = new ItemCollection($paginator->getAdapter());
    return $items;

I need something like this:

//$items = $paginator->getCurrentItems();

foreach($items as $key => $item){
    $items[$key]['_image_size'] = getimagesize($item['name']);

return $items;

But it doesn’t work. How to do that?