Aardvark Topsites PHP Manual

Word Verification

When members try to join your topsites list, they will be asked to copy a short series of letters and numbers from an image into a form field. This prevents the automated spamming that has been a problem for some list owners.

If the images don't work, then it is a problem with your server. You must have the GD library installed. If you do not, then you cannot use word verification. You can turn it off in the admin. If you are having troubles with spammers and word verification is not an option, you can try approving members before they are listed.