A HTTP header is a communication signal sent between server and client, it is NOT something you write into a page.
What I think you are asking about is a Document Type Declaration (DTD) or 'doctype' not a header.
http://php.net/manual/en/function.file-get-contents.php and http://php.net/manual/en/function.file-put-contents.php to save it.
$image = file_get_contents('http://www.url.com/image.jpg');
CTR = Click Through Rate/Ratio
It is the comparison or ratio of impressions (number of views for an advert) to the number of times visitors have 'clicked' through to the target URL.
How to improve it? .... Write better, more compelling adverts.
That is not advertising, it's just untargeted spamming of social media pages that is costly in terms of time and effort and hugely ineffective. I mean real advertising, buying Google Adwords for instance.