It's no surprise that Nelson Mandela's memorial service would produce a rain of stupidity and feigned outrage from conservatives over Barack Obama's behavior, since they see it as part of their purpose to police his every word, gesture, and blink for signs of transgression. The only appropriate reaction to this stuff (OMG, he shook Raul Castro's hand!) is probably mockery, but there is one thing that's worth a bit of consideration.
Whenever a bunch of world leaders get together and have time to stand (or sit) around and shmooze, there are going to be interesting photos that result. That's true even if nothing weird happens, like George W. Bush looking over at the most powerful woman in the world and saying to himself, "Hey look, a dame. I think I'll stroll over there and give her an unsolicited back rub." Any time we see powerful people just acting like people, there's something interesting about it. So when Obama, British prime minister David Cameron, and Danish prime minister Helle Thorning-Schmidt were photographed taking a little group selfie, it was bound to be noticed as one of those they're-just-like-us moments.
But because some photos of the moment show Michelle Obama with a serious expression on her face as she looks in the other direction, lots of people immediately fit this into a familiar story line, the one in which the First Lady is a castrating harpy always ready to smack her husband down if he gets too close to another woman, particularly a white woman.