I've finally figured out what's going on in this photograph:
What's the most important part of a meeting like this? The photo-op, of course. It's obvious that Professor Gates is trying to catch up to his friend, the president, so he can get the prime spot in the picture. So what happens? He's being held back (once again) by whitey. Sgt. Crowley is clearly pulling Professor Gates back by the elbow so he run ahead and be the one walking with the president. Perfectly clear to anyone with eyes in their head.