this is my guess, i dont know all the science though - when you close your eyes, you see nothing, well, black atleast, or i see a variety of red and green colours! whenever your eyes lack visual stimualtion, your subconcscious makes something up, in a bizzare way, its as if your eyes get bored. faces are something we as humans see everyday, all day, all the time, no matter where we are - unless obviously alone! :P so your imagination starts to make up things, and faces are easy for your brain to do!
another way of explainingf it is when people think they can see the face of jesus in something. stare at a random enough imagine that initially looks meaningless for long enough, and your brain will eventually dicipher something that looks like a face, or something similar. there is a scientific/psychological term for this...:( but i dont know what it is!