The surface of the Earth is whizzing by as Hubble orbits, and the pointing system, designed to track the distant stars, cannot track an object on the Earth. The shortest exposure time on any of the Hubble instruments is 0.1 seconds, and in this time Hubble moves about 700 meters, or almost half a mile. So a picture Hubble took of Earth would be all streaks.
To find images of Earth from other sources in space go to The Gateway to Astronaut Photography of the Earth: http://eol.jsc.nasa.gov/sseop/
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