Gallery

The current captcha used in gallery 2.2 is very simple in terms of the ocr
technology used by spammers. reCaptcha not only implements more anti-ocr
techniques, it also contributes to digitizing books. There is also a lot
of information on how to implement this in various languages. An issue is
that you must sign and get a key to use to help ensure you yourself are not
a spammer (this is all free). So there would be some setup involved before
activating it. Similar to how some other modules have to be configured
before activation. This would cut down on spam comments and also be
helpful at the same time

for more information see http://recaptcha.net/