Image functions are defined in catalog/image helper.

getOriginalWidth gives width of image.

getOriginalHeigh gives height of image. It is not getOriginalHeight().  Spelling mistake from Magento :- )

getOriginalSizeArray gives the height and width of an image in array.

$_product = $this->getProduct();

$imageWidth = $this->helper('catalog/image')->init($_product, 'image')->getOriginalWidth();

if($imageWidth > 600) {
	$lightboxImage = $this->helper('catalog/image')->init($_product, 'image')->resize(600);
}
else {
	$lightboxImage = $this->helper('catalog/image')->init($_product, 'image')->__toString();
}

// get height of image

// note that the function is getOriginalHeigh() .. not getOriginalHeight().. spelling mistake from Magento

$imageHeight = $this->helper('catalog/image')->init($_product, 'image')->getOriginalHeigh();

// get height width of image in array

$imageHeightWidthArray = $this->helper('catalog/image')->init($_product, 'image')->getOriginalSizeArray();