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');