I'm sure you've probably seen several 'Head in a Jar' tutorials on the net. Many of these can be pretty involved or have images already made for download to use.
I like the take used in the video below because it shows you how you can use your own face for this creepy prop. What makes this so cool is that it looks as if your face is smashed into the jar. The main component of this tutorial is to have a high resolution scanner/copier to make this happen.
The whole family can get in on the fun.
Check it out and give it a try.
If doing it yourself isn't for you or you don't have access to a scanner/copier, then click here to get a ready to download zombified face for your jar.