The image posted by
Weltraum in post #321 is a scan of a print. The edges of the print can clearly be seen at the top and bottom. Parts of a (logrithmic?) color control wedge are still visible.
This makes the image, which in itself is a composite, a multiple generation rendition. That, the low resolution, and jpg fragmenting totally kill any possibility of tracing anything meaningful.
Just from a purely visual check, the whole image is full of hairs, scratches, and irregularities, most of the latter following the radial composite lines, as mentioned above by Henna Oji-san.
Our famous smudge is on one of the radial lines and, strictly from an optical point of view, seems to be a coincidental overlapping of similar gray tones which have been produced by the resolution reduction and jpg compression.
There are other such smudges in the image, especially in the lower right part. Those are more linear. "Our" smudge is like the others, but seems to have blended with other gray values around it, resulting is something the human brain accepts as "anormal." A close up view shows that in this resolution there is a pixel pattern within the smudge that is consistent with the surroundings. Considering the image generation and resolution, that is of course meaningless.
Here's a version in which I highlighted a few hairs and other smudges with red circles.
http://img63.imageshack.us/img63/1921/thesmudge.jpg
One double smudge just northeast of the "polar circle" along the radial composite lines is interesting because two (almost) parallel lines appear to be present that look like football goal posts. I wonder what van Däniken would have made of them?