How do people in the Matrix develop a residual self image that resembles their own body

As we know people born in and connected to the Matrix retain a residual self image (Morpheus told Neo: "Your appearance now is what we call residual self image. It is the mental projection of your digital self."). Freed people that hack back into the matrix keep that projection and do not change it at will (see: Did People Within The Matrix Always Resemble their Real World Bodies?).

But here's the question: How do people born (or grown for that matter) in the Matrix develop a residual self image that resembles their own body in the first place? You simply do not know how you look if you never saw yourself. It would therefore require that the Matrix feeds your brain with a mental image based on your real body thus requiring some scanners (or whatever) inside the tank to obtain ones features within the goo.

That leads to the more important question as to why would the machines go through all this trouble to provide "proper" residual self images for their connected humans? The prime purpose of the Matrix is to keep their "guests" attached for live, pretty unaware of the fact that they are in the Matrix - as Morpheus puts it, blinded from the truth of being a prisoner. That goal could be perfectly achieved without everybody knowing his/her own look safe those who are freed from the Matrix (but then again why would the machines be worried about them)?

For the sake of the discussion lets assume that there is only one layer of the Matrix - that being: the "Zion-real world" is the real world - as nested layers of the Matrix would of course simplify things greatly.