Could the "X" be a kind of marker that the automated processing software adds to the picture to anomalous spots in order to later erase / smudge / hide?
One of the things I do when I am investigating a photograph is to find out what areas are pure black (RGB=0,0,0) as this means that I cannot get any further detail from these shadowed areas. On one of the photos I looked at recently, I found this, which I though was an amusing oddity.
Since it is only a bit of fun, I wont bother to give the full url as I do not think you will want to look at it. However if you do, it is Sol 2153 1P319319001EFFABCXP2372L5M1
