Cryptographp is a PHP library to create captchas. Activating this plugin create a captcha each time a user want to post a comment. Edit your comment template and insert <?php display_cryptographp(); ?> anywhere you want to display the captcha.
<?php display_cryptographp(); ?>
Default: 130/40
Default: 255/255/255
Default: 0/0/0
Default: 10
Default: 4/4
Default: 20
Default: 14/16
Default: 10/10
Default: 1/1
Default: 1