What are those pictures of? This one http://mars.jpl.nasa.gov/msl-raw-images/workspace/apss/ORT-1... Looks like a building with a garage door on it and some random pipes sticking out.
Meanwhile, if you look at the other photos with the horizon present (e.g. http://mars.jpl.nasa.gov/msl/multimedia/raw/?rawid=RLA_39750... ), the ground is dark because in order to capture the details in the bright sky, the exposure has to be adjusted so that less total light is captured (usually by using a faster shutter speed). Otherwise, you'd blow out the sky and get no details there.
It looks like the dynamic range of these cameras is limited, so keep that in mind when looking at any photos that come out of them. Something that's bright in one shot might be extremely dark in another, depending on its relative brightness compared to other parts of the scene. Take a look at the last few rear camera shots with the sun in them:
http://mars.jpl.nasa.gov/msl/multimedia/raw/?s=0
The sun causes all the sky detail from the previous rear camera shots to appear black, since its overwhelming brightness requires the exposure to be extremely brief.
Anyway, I hope that gives you a better idea of what to expect from these cameras and how to interpret the photos.