Questions about image cache

Adding the watermark to the & lt; meta & gt; header will not allow the browser to cache images. why?
The website uses thinkphp development, isapi_rewrite3 redirection URL, IIS6 server

However
1. IIS startup content expired
2. set header () in php ()
3.html Header
The browser cannot cache images. why?


How can we achieve this? even if the browser F5 refreshes, the image cache will not be cleared?
Such as this web site http://nanning.xjwy.cn/bao/img_show.php? Imgurl = http://nanning.xjwy.cn/upload_files/qb_bao_/1/87829_20130516110511_7bwar.jpg&baoban=

Only the image is loaded once, and the image is not cleared after refresh. Share To: header... 'data-pics = ''>
------ Solution --------------------
The page returned by the http://nanning.xjwy.cn/bao/img_show.php has

Of course, this image will be cached.
------ Solution --------------------
Add cache commands to img_show.php, for example:
Header ('cache-Control: max-age = 100'); // The number is the number of seconds you want to Cache
Echo file_get_contents ($ imgurl );
?>