Here are some possiblites of what the shroud is:
- it's the real shroud of Jesus with a Godly image of Jesus Christ on it
- it's the real shroud of Jesus with a latter-day man-made image of Christ on it, based on his real physiognomy.
- it's the real shourd of Jesus with a latter day man-made image of Christ on it, based on an artists impression.
- it's a shroud of someone from the time of Christ with an image on it which has later been proclaimed as the shroud of Jesus Christ in good faith (i.e. the person who proclaimed it, believed it was the shourd of Jesus).
- it's a shroud of someone from the time of Christ which has later been proclaimed as the shourd of Jesus Christ in bad faith (i.e. the person who proclamed it, knew it was not the shroud of Jesus).
- it's a piece of cloth from around the year 30AD which has been proclaimed the shroud of Christ in bad faith, and the image was later added also in bad faith (i.e. to convince people that it is the burial cloth of Christ).
As you can see, the authenticity of the Shroud depends on the different elements that make it up: the cloth itself, the image on it, the honesty of the people who handled the Shroud.