From what I could tell by what was on TV (captions) it is a different cultural than those of us in the US are most familiar with. It was explained as a celebration of his life and anything but somber.
As far as the picture is concerned they discussed it on the Today Show and had information from the photographer that Michelle had been joking with them moments before but by the instant the photo was taken something different had caught her attention.
That was my understanding. I have heard the whole stadium was in some kind of euphoria, so some behaviors can be overlooked.