Android version of BlackBerry 10's Picture Password unlock method (don't sue me!)
Here's a nice video!
You will have to build it yourself with the Android Developer Tools in order to use it. Once you're done, you have to install it as a system app, overriding Face Unlock. You will be able to enable it in Settings > Security.
WARNING: This is experimental software. It has only been tested on two devices. I cannot guarantee it will work for you, and I cannot guarantee it won't lock you out of your device. I also cannot guarantee it will not make your phone burn. If you decide to use it, you agree that I will not be held accountable for any damage arising from the use of the software.