Well here is my thinking.
There are only a couple of files that call the captcha function.
Under images is the background images and ttf files.
Usually if there are any extra files other than the jpg or ttf files it causes a problem with displaying the image.
Does not appear to be the case here.
Then you have a captcha.inc.php file in the libs and script folder.
They could be corrupted of do not have the correct permissions.
Then of course is you have two options to use either gd or IM.
So one of them may be the culprit so you could change that in the registration settings.
If this is the case then I would assume you would have issues in a gallery forum also.
Other than that instead of playing post tag. Open a support ticket.
