Several things might cause CAPTCHA images to not appear:

- the path and/or URL to the CAPTCHA directory is not specified correctly under Admin > Captcha Preferences

- images/captchas/ directory (or whichever is specified) is not set to 777/writable

- the server does not support True Type Fonts (try turning the option off under ‘Admin > Captcha Preferences’)

- GD library isn’t installed and/or working correctly

- system/lib/words.php file doesn’t exist