sorry, let me elaborate.. you're seeing one picture, basically, the famousest one, that's it, from one angle, you can do that with any picture once you've found a good angle. So it's not even simulacra [which at least can be viewed from several angles, often many.] So it's an illusion; think how many alternative pics of it they probably threw away to enhance the illusion. Now the man in the moon.. that's a whole different ball game...
Some years ago I read that the human brain "looks for" images it can relate to. When looking at a pattern in curtains for example, the mind sees a face in the pattern which is not actually there. This applies to cloud formations, the Moon and in this case, Mars etc.
On a similar note to robber1's answer; it has been said that you will see what you are told to see, so, if someone tells you it looks like a face it would be very difficult to look at it as anything else. It is weird though!