Well, they have haven’t they? I think you need to put yourself in the shoes of a believing Christian/catholic. There is really no need for them to try and explain scientifically what is explained miraculously. From the miraculous POV the act of resurrection itself may have created the image and since man cannot replicate that process, it’s impossible to explain it scientifically. But what we do know is that certain energetic particles applied to certain mediums do leave traces and can leave negative images. So even science helps to lend credence to their theory of a miracle
So if it is an ancient piece of cloth and there isn’t a scientific explanation yet that adequately explains how the image could be produced and since you cannot prove that a God does or doesn’t exist…a believing Christian can easily rely on faith that it is what it is purported to be.
So it’s up to you to prove them wrong, not up to them to prove that they are right.