This image shows the number of people per square kilometer around the world in 1994. The data were derived from population records based on political divisions such as states, provinces, and counties. The image was taken from here.
Converted from TIFF to PNG with tifftopnm and pnmtopng, preserving alpha channel.
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The NASA website hosts a large number of images from the Soviet/Russian space agency, and other non-American space agencies. These are not in the public domain.