I noticed there is a bug report on invalid links to captcha images in the guestbook form.
see: http://joomlacode.org/gf/project/jambook/tracker/?action=TrackerItemEdit&tracker_item_id=13380

I just wanted to share my solution: it checks if the filepath is ending with a slash, if not add the trailing slash. Works for me.

file: components/com_jambook/ocr_captcha.class.php
lines: 103-110
new code for functions get_filepath and get_fileurl

function get_filepath($public="" ) {
return (( '/' != substr($this->server_root, -1))?$this->server_root.'/':$this->server_root) . $this->_DS . $this->get_filename($public);
}

function get_fileurl($public="" ) {
return (( '/' != substr($this->server_url, -1))?$this->server_url.'/':$this->server_url) . $this->_DS . $this->get_filename($public);
}

Regards,
Gerrit

That code wouldn't actually fix anything.

The $this->_DS is already adding a slash, so why would an extra be needed?

Granted, the code should probably check that an extra slash isn't added, but two wrongs doesn't really make one right.