dayvead on "Captcha On Login Form"

Good afternoon,

May be i might have missed something, but i'm trying to put a captcha form on my login form .
I've done the next steps :
.) Activate the extension zpCaptcha in the extention panel
.) Made a few changes in the option panel for this extention

But obviously nothing appear.

So my idea is that i sould put some code in password.php, using this function : getCaptcha("")
But for sure at this time i couldnt find any tutos. or documentation.
( Version Zenphoto 1.4.5
Version PHP : 5.3.16
Version PHP GD library : 2.0.34 compatible
)

Thanks a lot anyone for replying