magento add programatically gallery image to existing sku

<?php
error_reporting(E_ALL);
ini_set('display_errors', 1);
require_once 'app/Mage.php';
Mage::app();
Mage::app()->getStore()->setId(Mage_Core_Model_App::ADMIN_STORE_ID);

$importDir = Mage::getBaseDir('media') . DS . 'import' . DS;
$productsData = array('2abc.jpg','2def.jpg');
$productSKU = 'SKU';
$product = Mage::getModel('catalog/product')->loadByAttribute('sku',$productSKU);

foreach($productsData as $fileName){
    $filePath = $importDir.$fileName;
    if(file_exists($importDir . $fileName)){
    $product->addImageToMediaGallery($importDir . $fileName, null, false, false);
    echo "done";
} else {
        echo $productSKU . " not done";
        echo "<br>";
    }
}
$product->save();
?>