Do read Sight and brain: an introduction to the visually misleading images by Daniele Murra and Paolo Di Lazzaro:
. . . We should consider this “subjectivity risk" when using computer tools to elaborate images, because we may generally have the propensity to make visible something that we want to see but that is not embedded in the original image.
Concerning the scientific approach to the acheiropoietos images, only reproducible experiments are scientifically acceptable.
Interpretations of shapes, coins, faces, flowers or letters “seen” on acheiropoietos images by means of image processing tools should be considered a track useful to address further studies, but they cannot be considered as self-consistent proofs.
It is a short paper, wonderfully informative and easy to read and understand.