A microscopic photograph (200x magnification) of image fibers from the tape samples of particles collected by Max Frei. Photo is by Kevin Moran. 

In an article, "Optically Terminated Image Pixels Observed on Frei 1978 Samples," Kevin Moran describes the image pixels:

The individual image pixels have very sharp boundaries at their ends across the 15-micron diameter of the fibers. When seen at a magnification of 200 power, these pixels show uniformly darkened area over the natural color of the non-imaged fiber. At the boundary between the image pixel and the clear fiber, there is a sharp change. There is no gradual edge as expected from a shadow mask or external light source...

The fact that the pixels don’t fluoresce suggests that the conversion to their now brittle dehydrated state occurred instantly and completely so no partial products remain to be activated by the ultraviolet light. This suggests a quantum event where a finite amount of energy transferred abruptly.

Physicist John P. Jackson, has shown that the image appears to be perpendicular to gravity. There is no sideways or angled image formation. For instance, there is no image of the top of the head. The image also seems to exist only where the cloth was within 3.5 centimeters of the body.

The image appears darker where the cloth was closer to the body. Thus, proximity of body and cloth results in more pixels or longer pixels. 

One explanation, that some scientists have offered, is that some, yet unknown, form of  radiation was emitted from the body with enough intensity to chemically alter the fiber. Kevin Moran states it this way:

The unique front-and-back–only image can be best described as gravitationally collimated. The radiation that made the image acted perfectly parallel to gravity. There is no side image. The radiation is parallel to gravity and, if moving at light speed, only lasted about 100 picoseconds. It is particulate in nature, colliding only with some of the fibers.

It is not a continuum or spherical-front radiation that made the image, as visible or UV light. It is not the X-ray radiation that obeys the one over R squared law that we are so accustomed to in medicine. It is more unique.

It is suggested that the image was formed when a high-energy particle struck the fiber and released radiation within the fiber at a speed greater that the local speed of light. Since the fiber acts as a light pipe, this energy moved out through the fiber until it encountered an optical discontinuity, then it slowed to the local speed of light and dispersed.

Other scientist disagree and suggest a Maillard chemical reaction between heavy amines produced by the body and the carbohydrate layer coating the fibers. See Tough Questions: Chemistry of the Image.