Aha! You can see the shadow of the camera, to the top and left of the nearest wheel. There are no shadows leading away from it. You can see from the sundial that the sun is almost directly overheard, meaning that if there was a person taking this picture, you'd see the feet near that shadow.
Furthermore, it would be absurd to think any team of hoaxers would be so stupid as to allow anyone on site lest they leave footprints behind. It really isn't hard to see that these people, whether professional hoaxers or scientists, know how to do some clever things with this robot and some simple editing techniques - i.e. polygonal marquis tool plus copy and paste, and faded edge. It is not a complicated trick.