Unfortunately, the kind of widget you're looking for doesn't exist in Website Maker at this point. The closest thing to it is probably the guestbook, but I understand that this is not really what you're looking for. While you could have several instances of the guestbook on one website, they would all draw from the same database and thus show the same content. Therefore it is not really suitable to be used as a comment box widget under multiple photos. However, if you're really just looking for a simple guestbook/shoutbox widget, it should do the job.

If you're looking for a full-fledged forum component, I can only recommend setting up an externally hosted forum system with one of the countless (free) providers out there and linking to it from your website to open in a new window.