Can the iSight camera perform facial recognition at the lock/login screen?
Solution 1:
KeyLemon looks like what you want: http://www.keylemon.com/product/
It has a free version, but I don't believe it will replace the login screen.
Solution 2:
I'm not aware of any software that will allow you to do that.
I would comment on the KeyLemon suggestion, but I don't have enough rep!
I do not think this will replace the standard OS X login screen. It looks like it will add another, working like a screensaver, to restrict access to your machine once you have logged on.