alt Hacker NewsA copyrighted image of a nude model elected for no obvious reason has a test image in the University of South California by some pervs and then used in a lot of papers as a test image.
Or, a standard cropped image of a playgirl used in the field of image processing.
"elected for no obvious reason" isn't quite right, as a test image for computer graphics it has regions of very high frequency detail and regions of very low frequency detail which make it easier to spot various compression artifacts, and it makes a good study for edge detection, with both very clear edges along the outline, but more subjective edges in the feathering.