It’s a piece of dirt on the lens. If you go to the Yahoo Photos gallery, it persists in the same spot in photos 4-6 despite those photos taken of entirely different subjects.
Now the question is - who squeegeed the lens clean for photo #7!!!??
... it would have to be underneath Curiosity for this one.
And how about that penny! Check out the penny. You can see the dust that settled on Curiosity from the landing. In fact, in that reprocessed panorama shot I thought I could see a large cloud of sorts that was still persisting.