Open view_video.phpand put this code .

PHP Code:
$html_head_extra = '<meta property="og:image" content="' . VSHARE_URL . 'thumb/' . $video_id . '.jpg" /> ';
$smarty->assign('html_head_extra', $html_head_extra); 
This will post the exact image of teh video.
Also you can put other code at $html_head_extra.