So, the more plausible explanations so far are
1...Dust on the lens...not likely because of the shape.
2. Digital screwup.........
3. Some sort of marker they use for measuring
Anyway here is the original pic....right from Nasa
And a link for those who like to click
http://mars.jpl.nasa.gov/msl-raw-ima...CAM00295M_.JPG
