A few people have recently had issues with the image captcha. This has been due to a variety of reasons including lack of GD support and problems with sessions. As an alternative a new add-on has been released called ReCaptcha. ReCaptcha is widely used, secure, free, helps to digitize books, and best of all it doesn't require the GD Library or a session to work. To use ReCaptcha you will need to sign-up at their website, http://www.google.com/recaptcha (you can log in with your Google account if you have one). If it's popular it may be added to the next version, either as an alternative or a replacement to the current captcha, so feedback is very welcome, even if it's to say that it works. You can download the add-on on the Add-ons page.