That has always been my problem. Try wrapping a bust in a paint soaked rag. Upon unwrapping, you're looking at an extreme distortion of the facial and other body features. So, the energy emanating from the body only came out the front? I don't get it.
Having said that, I've seen suggestions that the image could have been formed by a sudden burst of intense energy that emanated outwardly in every direction and "scorched" (for want of a better word) the Shroud in a manner in which the intensity of the "scorch" at any given point was indicative of the distance between that point and the surface of the Body's skin. Therefore, the points where the image is darkest (or lightest, in the negative) are those where the Shroud was in actual contact with the skin ... while the points where there is little or no color in the image were not in direct or close contact with the skin.