Securimage-WP-REG adds captcha ptotection from Securimage-WP plugin on register pages.
Unfortunately, it didn't have option to be installed on user register page. Now you can just install Securimage-WP-REG, enable it - and you will have CAPTCHA on register page!
Securimage-WP-REG adds captcha on register pages and fixes one little bug of Securimage-WP. I hope it will be fixed in future and I added option for it.