This happens at the moment due to a security feature that we added to all Joomla and Wordpress sites after attacks on those types of site became more common. These attacks are brute force attacks aim at the admin pages of these sites attempting to gain access. The system now asks the "Are you human?" question to any IP that has logged a large number of requests to the page in a short amount of time.
This 'Captcha' system blocks access to the bots used in the attacks, as they are not able to read the image code and enter it into the field to get past the security check. This keeps your site safe, as the bots are not able to try and force their way into the admin area, and also prevents our servers from getting overloaded.
