is your gb-folder (i mean the folder on your webspace, where the files of your guestbook are located) a subfolder of your webpage?
I had the same problem with my gb on my hoster 1und1.
There i located my files so:
But I configured my gb as subdomain pointing on /gb (I mean www.domain.de pointing on /web and gb.domain.de pointing on /gb)
To see the captcha-code I had to change the base_URL to my system-domain (s????????.online.de)
Thanks for your reply, Torsten. My gb-folder is a subfolder of my domain webspace, i.e., it is www.mydomain.com/gb/index.php. I was unable to find why it doesn't work properly, so I changed to lazarus script, which it's basically the same script (a little bit better), but it works fine.
I'll try it again following your instructions and I'll see ...