It does look like his pants legs are two different colors. That could be just the pants he's wearing.
But say it is photoshopping. Two questions come up.
1) Why photoshop the guys legs? Presumably Obama would be the only person a fraudster would be concerned with -- or are all the heads photoshopped in? If so, why? If someone in Africa could recognize the family as a whole or its members individually, would they be deceived if other heads were swapped in?
2) To what end? Presumably a family might have wanted to claim Obama as a relation, but what would Obama get for it? Does this photo of him with an African family in some way create a mythic past for him? Does it advance further Communist agenda? Haven't the Obamas in Kenya already acknowledged him as a blood relation? So what's the point?
If you look at any photo long enough, things will start to look fishy. And that's especially true in third world countries. But what nefarious purpose would be advanced by such a forgery?
I wondered the same.