Like I ask above, I have custom login (and registry) that created by Profile Builder Plugin.
The problem is the captcha plugin (perhaps) cannot auto detects my custom login form, so the captcha won't appearing.. But instead of this, when I try to login there the error message always shows up as saying I've entered the wrong image...
http://wordpress.org/extend/plugins/sweetcaptcha-revolutionary-free-captcha-service/