How to get this working on your site:
Once you have added the code to your PHP pages, you will need to edit captcha.class.php file.

var $imageDirectory = '/path/to/your/captcha/image/directory';

var $imageURL = 'http://yourdomain.com/captchaImages';

var $captchaLabel = 'Please complete the Captcha:';

var $captchaBoxStyle = 'border: 1px solid #ccc; padding: 3px; margin: 3px; width: 210px; font-family: verdana; font-size: 11px;';

		

Explaination of these variables
$imageDirectory: This is the path to the directory that the images will be creates, this needs writable permissions: chmod 777.
$imageURL: This is the URL to the images directory so we can output them to the users.
$captchaLabel: This is what is displayed above the Captcha telling the user what this is for.
$captchaBoxStyle: This is the style of the captcha box.

Now for the download link: Download it here