BuddyPress.org

I tried captcha technique. but some how its not working with my installation. I dont know what the exact reason is. The image is not get displayed. when i clicked on the test link at the Si Captcha settings page, it is redirected to “not found page” ..

But Buddypress test site (testbp.org) does not hold a captcha.. but still it seems that there is not much spam registration. How they control spams without using captcha. Any idea about it?..