Soothe, if you look closely, you can see that the formation you posted is not the only one that shows the tram lines.
I am almost certain that these images are real photographs, because I have seen a couple of these patterns from other angles, in other photos. I've even seen one of them on a TV program about crop formations (not a still photo, video).
As to why the would choose grain as a medium of communication I don't know. Maybe it is a way of slowly introducing us to the idea that there is indeed other intelligence 'out there', while still allowing those that can't/won't believe it to say, 'well it's obviously human made' and go back to reading the bible. People could say the same thing with stone if you think about it... As to how they would carve something in treetops, well you got me there
Why don't they just land on the whitehouse lawn you could ask? Could you imagine the reaction that would produce in millions of people around the world? People would have their entire belief system turned on it's head on an instant. By slowly introducing the idea, the impact might not be as extreme.