I literally just did this!
Add this: https://ww.wp.xz.cn/plugins/si-captcha-for-wordpress/
You can have it display only on registration, or on login too. Works like a charm.
Pic: http://www.dropshots.com/sydneymc/date/2015-02-25/11:51:56
thanks for the suggestion site4pets;
Install GoodBye Captcha – works like a charm for me and… no need for captchas.
https://ww.wp.xz.cn/plugins/goodbye-captcha/
