Ok, look, guys.
The image looks somewhat like a rodent, so it could be an actual rodent, or just a rock shaped like one, with light falling on it giving this impression. Both of these are possibilities when looking at the photo.
But since the photo was taken on Mars, I'm guessing it's not an actual rodent. It could still be, and in that case, joy to the world, we have first contact with an alien race.
Well, they are trying to argue that it just proves that the photo was really taken on Earth. At least I think that's what Tom's trying to say. What is the FE perspective on Martian fauna, anyway? Maybe that's better left for a separate topic.

However, although you could say that these are all possibilities:
1) Martian rock that happens to superficially resemble a squirrel when viewed from a certain angle under certain lighting conditions
2) Actual Martian squirrel
3) Earth squirrel photobombing a NASA conspiracy photo taken out in a desert somewhere
I think you can see that the 3 don't necessarily have
equal probability.
For #1, pareidolia is a well known psychological phenomenon that probably everyone here has experienced. The photo is a perfect place to experience this, due to the multitude of little shapes in it. So it seems extremely likely that you could see something like a little rodent, which are already evolved to blend in to their surroundings.
#2, well if you believe that we have been to Mars, and you look at the experiments and data collected over the last 30 years or so of Martian probes, rovers, flybys and satellites, it seems unlikely that we'd have missed an example of Martina fauna of this size. Right now it's hard to tell if there were ever even microbes, although the evidence seems to show that it was at least possible at some time in the past.
#3, the vast conspiracy at least to me is way too far fetched to be real. I'd have to seriously doubt my own mind and observations of the world to accept it, so it would be very un-Zetetic of me to give this one more than a slight chance.

Also, if you think this is an Earth squirrel, I would like to see somebody identify its species. Although squirrel-like it looks odd to me, I have never seen any squirrel just like it.