Hello, I released a module that provides an image-recognition CAPTCHA:
https://drupal.org/project/ir_captcha
I would be glad for any feedback or contribution.
An image-recognition CAPTCHA shows a bunch of images and asks the visitor to select certain ones. For example, the visitor might be asked to select the images with trees.
The module comes with only a few example images. The intended usage is that the administrator provides their own images and questions. This is more work, but allows creating a CAPTCHA unique to the website. Having a unique image-recognition CAPTCHA is important because image-recognition CAPTCHAs can be defeated using image classifiers. The theory, though, is that the work it takes for a spammer to train a classifier to defeat a unique image-recognition CAPTCHA for a single website would not be worth the payoff (for most websites). There is guidance in the README.txt for finding images to use.
The module has no dependency on external web services. That is different from the only two other active modules on drupal.org I found that provide image-recognition CAPTCHAs: reCAPTCHA and Confident CAPTCHA. To me this has pros (+) and cons (-):
+ Not reliant on those services
+ No credentials or API key to manage
+ Able to use your own, unique images
- Have to do the work of finding/installing images
There is an alpha release for Drupal 7. I have just begun working on a Drupal 8 port.
[Note on naming: The CAPTCHA literature calls the type of CAPTCHA provided by the module an "image-recognition CAPTCHA", which is why I chose the name "Image-Recognition CAPTCHA". This is unfortunately a bit similar to the name "Image CAPTCHA", which provides what the literature would call a text-recognition CAPTCHA. I felt it better to be consistent with the literature than to invent a new term.]