function getStyle($url) {
	$size = getimagesize($url);
	return (int)$size[0] > (int)$size[1] ? 'height: 100%;' : 'width: 100%;';
}

echo getStyle("https://pp.vk.me/c627723/v627723406/1b5c5/OfbayIeP89U.jpg");