$product = Mage::getModel('catalog/product')->loadByAttribute('sku', $sku);
$productImgUrl = $context->helper('catalog/image')->init($product, 'image');
$productImgSmallUrl = $context->helper('catalog/image')->init($product, 'small_image');
$productImgThumbUrl = $context->helper('catalog/image')->init($product, 'thumbnail');
