But, as AFP's Roberto Schmidt pointed out in a blog, "photos can lie."
Writes Schmidt: "In reality, just a few seconds earlier the first lady was herself joking with those around her, Cameron and Schmidt included. Her stern look was captured by chance."
Schmidt also discloses that Thorning Schmidt, the Danish prime minister, produced the smartphone in which the selfie is now presumably stored.
The photographer describes the incident, which occurred just after Obama's eulogy for Mandela:
"So Obama took his place amid these leaders who'd gathered from all corners of the globe. Among them was British Prime Minister David Cameron, as well as a woman who I wasn't able to immediately identify.
"I later learned it was the Danish Prime Minister Helle Thorning Schmidt. I'm a German-Colombian based in India, so I don't feel too bad I didn't recognize her! At the time, I thought it must have been one of Obama's many staffers.
"Anyway, suddenly this woman pulled out her mobile phone and took a photo of herself smiling with Cameron and the U.S. president. I captured the scene reflexively."
Schmidt also pointed out that this was a memorial service, not a funeral: "All around me in the stadium, South Africans were dancing, singing and laughing to honor their departed leader. It was more like a carnival atmosphere, not at all morbid. The ceremony had already gone on for two hours and would last another two. The atmosphere was totally relaxed -- I didn't see anything shocking in my viewfinder, president of the U.S. or not."