I tried to remove any code pertaining to that both in the header insert and in the body tag, but it did not resolve the problem.
A bit more info:
Looking trough the actual output on the page, Firefox and Opera simply embeds
../images/general/default/blank.gif where the CAPTCHA image should be output.
WebKit browsers, however, output
ubbthreads.php?ubb=captcha&id=<sessionid>&t=
which, when clicked, produces
ubbthreads.php?ubb=captcha&id=<sessionid>&t=gd
Edit: Or rather - that's the images' link.