Do I miss something, I am not able to see, why pil is needed...
PIL is needed for the CAPTCHA.
how do I enable captcha or when is it triggered?
The CAPTCHA is automatically enabled. It triggers when the ratio of text to links is too high, it becomes unrealistic. Just try to paste a bunch of links, e.g. "http://spammer.domain.com", and it'll pop up the CAPTCHA.
Just as Anonymous said, it's activated by default and PIL is used for the Captcha.