File:MarsCuriosityRover-Penny-20131002.jpg
The previous highest-resolution MAHLI images, which were pictures of Martian rocks, were at 16 to 17 micrometers per pixel. A micrometer, also known as a micron, is about 0.000039 inch.
The penny is a 1909 VDB penny minted in Philadelphia during the first year that Lincoln cents became available. More information about its inclusion on the MAHLI calibration target is available at http://www.jpl.nasa.gov/news/news.php?release=2012-033 .
The gold medal for highest resolution photographs on Mars goes to NASA's Phoenix Mars Lander's optical microscope. As a microscope, though, fine-grained samples had to be delivered to it, whereas MAHLI can be deployed to look at geologic materials in their natural setting.