Download the url to a local temp file and then process it with getimagesize so we can optimize browser layout

file.php

<?php

$img_url = 'http://someurl.com/image.jpg';

if ( !empty( $img_url ) ) {
	$tmp_file = download_url( $img_url, 10 );
	if ( !is_wp_error( $tmp_file ) ) {
		$size = getimagesize( $tmp_file );
		$img_width = $size[0];
		$img_height = $size[1];	
		unlink( $tmp_file );
	}
}