I have a similar problem currently that I'm trying to work out, and one way to help debug is to have warnings turned on, and then to take a captcha url and execute that directly.
What I've found is that the problem lies in my freetype support.-- it's not working for some reason.
Here's an example:
http://www.gizmola.com/blog/plugin/capt ... ff43c53579
Running this shows:
The image cannot be displayed because it contains errors.... etc
Viewing source on the page, I note the problem:
<br />
<b>Warning</b>: imagettftext(): Could not read font in <b>/sites/gizmola.com/blog/plugins/serendipity_event_spamblock/serendipity_event_spamblock.php</b> on line <b>750</b><br />
<br />
<b>Warning</b>: imagettftext(): Could not read font in <b>/sites/gizmola.com/blog/plugins/serendipity_event_spamblock/serendipity_event_spamblock.php</b> on line <b>750</b><br />
<br />
<b>Warning</b>: imagettftext(): Could not read font in <b>/sites/gizmola.com/blog/plugins/serendipity_event_spamblock/serendipity_event_spamblock.php</b> on line <b>750</b><br />
<br />
<b>Warning</b>: imagettftext(): Could not read font in <b>/sites/gizmola.com/blog/plugins/serendipity_event_spamblock/serendipity_event_spamblock.php</b> on line <b>750</b><br />
<br />
<b>Warning</b>: imagettftext(): Could not read font in <b>/sites/gizmola.com/blog/plugins/serendipity_event_spamblock/serendipity_event_spamblock.php</b> on line <b>750</b><br />
I'm doing some work on my gentoo install, and will explore this further when things are stabalized there. This smells like a gd/freetype/php configuration installation issue.