Make Sure "Use Captcha" is enabled in Admin > Extensions page by editing the Default Authentication options.
If you are unable to see the captcha image after properly enabling Captcha for registration or for those modules which support it AND IF AND ONLY IF your site is running in an ASP.Net 4.0 application pool, you will need to make modifications to web.config to allow Captcha and the LinkClickHandler and other handlers to properly load:
1. Open web.config for editing.
2. Locate the <handlers> section.
3. Within the <handlers> section there are a number of <add nam =" . . ." . . . /> nodes.
4. On all of those which include the following attribute and value
change the value to
5. Re-save and if necessary re-upload the modified web.config file.
Solution Provided by: William Severance