The easiest way to do it in details.html template. Try add:{if user_loggedin}<img src="{template_lang_image_url}/postit.gif">{endif user_loggedin}{if user_loggedout}<img src="{template_lang_image_url}/postit_off.gif">{endif user_loggedout}
If you need also create a link for that button, in details.php find: "msg" => $msg,
Insert below:
"url_postit" => $site_sess->(ROOT_PATH."postit.php"),
(asuming you need a link pointed to postit.php file which located in your 4images root directory)
Then you can use {url_postit} tag for the link.