"So, here we have a number of possible explanations:"
1) Speck of dust in the air
my photography experience is limited but a speck of dust wouldn't even register on any lens I've ever used. Too close it's a blur, too far away it's too small, and 'just right' would leave the rest of the image totaly out of focus.
2) Data error or transmission error
It's not like we'd seen previous reports of random dropped pixels.
If they had been a problem we'd at least have seen one or two out of the large number of images avalable to us. (This alternative requires either a very, very, rare random event, or first sign of 'new' system degeneration. The latter might be a possibility but either would require repetition.)
3) Phobos, Deimos, or another trapped asteroid
As someone else noted, NASA knows where these bodies are at the time the photo was shot; there are then three alternatives, [a] NASA ignored a historic phorto because NASA does not need or want positive PR, [b] NASA is preparing a huge PR release to tell us about it right after the Rose Bowl game, [c] NASA doesn't know any better than we know what it is.
(which seems the most simple to you?)
4) Alien vehicle
Much like alternative number 2 - can't be proven or disproven as yet.
Going back to the original photo, with the landing mechanism directly in front of the rover. My second alternative goes away - so how many similar errors have been noted during all of our reporting on the little guys?