To put a more cynical note to it, the second picture (with the Americans leaving towards Mexico) should have a wall built at the border - just like the famous Berlin Wall - because this wall was initially build by the GDR to not allow well-educated people to escape the country ...
Maybe the other leaders played a prank and all planned on waving at the same time and then left Mr. Obama waving by himself, while they snickered amongst themselves towards the camera.