Huh. Not really what I was expecting!
The image above was discovered by Test Your Might, and it's the product of PC players tinkering with the game. While Jason Voorhees does wear a mask during the entirety of Mortal Kombat, Netherrealm Studios did render a face under there. A face that nobody was ever supposed to see.
Here's what Jason actually looks like in the movies:
Picture: Evil Geeks
I think I prefer the Bruce Willis version of Jason.