For this tutorial I'm going to use Dani from Cradle of Filth because he's too awesome.

1. Open up the picture you want. In my case, it's Dani.

 
2. Go to Colors on the menu bar.
   
    

3. Go to Decrease Color Depth under Colors.
   
    

4. Click on 2 Colors (1 Bit).
   
    

5. It will take you to a window that looks like this.
   
    

6. Copy the settings from the above image, then click ok.

7. Your image will be reduced to one layer that looks kinda funky.
   
    

8. Press Control + C to copy the image.
    

9. Then press Control + Z to return the image to the original state.
    

10. Last, press Control + L to paste the funky image on top of the original image.
     

11. Fool around with your transparency to the funky image until it looks like whatever you want it to look like.
     

12. Your image should come out like this.