When held up to a light, "Raising the Flag on Iwo Jima" overlaid on a U.S. map is visible. Under normal lighting, the image is not visible.
This is done by converting an image from grayscale to a 3D contour map. Thicker sections of the print are less transparent and darker when held up to a light, whereas thinner sections are more transparent allowing light to shine through.
This file is meant to be printed in using Stereolithography (white) due to its fine feature detail and need for semi-transparency.