Ohhhh! I know what your talking about now, that little dark spot in the photographs. Oh man, this is toooooo funny. Time for a justified rare instance of l33t speak on wikipedia: ROFLMAO!!! 
regain compusure* K, let me explain.
I'll start with the basics: spacecraft are very expensive. They can only send a few out of earth orbit (where they have no ability to generate income like earth sattelite spacecraft) every so often. For various reasons they place the few spacecraft that can be afforded in equatorial orbitsmainly because they can't afford to send out 1 spacecraft for each planet, so they have to keep the sattelite orbiting in the solar plain so it can be sent to another planet later. From equatorial orbits there is only one way to take a picture of the polesfrom the side! Even though the picture looks like it is one shot from the top, it is actually several shots from the side and put together to make it look like its one shot from the top. In order to make it look like its taken from the top, they have to correct for the fact that its actually taken from the side. The very top just becomes a gross stretched out mess. 
You probably don't notice, but there is a pattern here of seeing fantastic explanations where there is a simple explanation. Its a certain type of thinking that tends to make people prone to conspiracy theories.