PHP

list($width, $height, $type, $attr) = getimagesize('http://davidvandertuijn.nl/assets/images/david-van-der-tuijn.png');

JavaScript

function getimagesize(url){
	var image = new Image();
	image.src = url;
	image.onload = function(){
		alert(image.width + 'x' + image.height + ' pixels');
	}
}

getimagesize('http://www.davidvandertuijn.nl/media/images/davidvandertuijn.png');