So far the only probe to visit Neptune has been the historic Voyager 2 mission.
This picture was taken 6.1 million kilometers from the planet and shows the great dark spot which was discovered by Voyager but which according to a Hubble photograph has since disappeared
Closest approach took place on 25th August 1989
The planet was found to have a magnetosphere highly inclined to the planets spin axis and an active atmosphere, which was not expected for a planet so far from the sun
Voyager also imaged the moon Triton and discovered six new moons including Proteus