“The image could have been added centuries after the linen was first woven.”
True, but efforts to produce a copy have failed miserably.
Why worry? If you believe its true—to you it is a true artifact. If you don’t? why worry about it? God will never let us know for sure because that would take from us Free will, his greatest gift to mankind. Ever wonder why God loves us more than he loves the angels? The Angels see and experience God and still have doubt. People believe without seeing him and trust in his love.