Here a quick and easy way to change the thumbnail image that Facebook picks for your website when a users clicks on the Like button or shares a link of your website on Facebook; Add the following line of code in HEAD section of the page:
where THUMBNAIL_IMAGE.JPG is the URL to the image that you’d like Facebook to use a the thumbnail.
To clear the browser cached page or to force the browser to re-download the content of a page, you can place the following HTML code in the header tags (<head>) of the page. This code will ask the broswer to ignore any saved copies of the page, and to re-download page content.
If you prefer working with PHP, you can do the exact same using this PHP code:
header ("Expires: ".gmdate("D, d M Y H:i:s", time())." GMT");
header ("Last-Modified: " . gmdate("D, d M Y H:i:s") . " GMT");
header ("Cache-Control: no-cache, must-revalidate");
header ("Pragma: no-cache");