Assign existing media gallery image to the loaded product.

magento-assign-image.php

$product = Mage::getModel('catalog/product')
            ->setStoreId($store->getId())
            ->load($product->getId());

$gallery = $product->getMediaGallery();
$mediaAttribute = array('image', 'small_image', 'thumbnail');
if($gallery && count($gallery['images'])) {
    $product_attributes = $product->getTypeInstance()->getSetAttributes();
    $gal_backend = $product_attributes['media_gallery']->getBackend();

    foreach($gallery['images'] as $image_data) {
        $gal_backend->setMediaAttribute($product, $mediaAttribute, $image_data['file']);
    }
}
$product->save();