The eyes are not connected straight to the main brain, they feed into a 'frame buffer' which creates an image. The brain then looks at what is in the 'frame buffer' and can also add/feedback into it.
This is partially why psychosis can seem so real. One part of our brain feeds into the 'frame buffer', the other part of the brain interpreting what you see can't tell the difference between input from the eyes and feedback from the brain. Those spiders you are seeing are as real to you as if you really do see them with your eyes.
This also goes some way to explaining how the 'gorilla walking through the basketball game' illusion works and other visual tricks such as not seeing the blackout when your eyes move - you really didn't see that cyclist etc.
(I've explained that in my simple language from recollection as it's how I easily understand it.)
Same with faces: I guess nobody can really 'see' a face in their imagination, at least not for more than a few seconds of high-resolution short-term memory. They can, however, use their knowledge about nose shapes etc. to construct a symbolic model of the face in their mind.