Go to their website and click on the big green MAKE ME ZOMBIE! button.
On the next screen Choose File, and find the picture you want to use.
You’ll get a little pop-up in the middle of the screen (Play Across Zombieland). Play if you want or just click on Skip to go directly to your picture.
You will be taken to your zombiefied picture. Click on Save Image to save it to your computer.
I was going to wait for Halloween to write this… but… MUST EAT BRAINS NOW.