Press photographers don't generally have too much control over what caption is put under their pictures. In the case of pictures like this, in a situation like that, it seems extremely unlikely that any photographer or reporter would have been in a position to gather and pass on the kind of specific information ddw posits.

Far more likely it would have been a matter of people in news agency offices far away from New Orleans writing captions in a hurry on the basis of what they see in front of their eyes, and how they interpret it.

Wolfgang has explained the point he was making pretty straightforwardly. It's not a question of calculating distortion, carried out by people with some racist agenda. It's that people are unconsciously predisposed, by all kinds of factors, to make assumptions about what they see with their eyes, and stereotypes about colour can come into that.   This was an example of how it can work.

I think it is fair to say that, presented with those two pictures without any caption, a lot of white people, who are in no way consciously racist, would be likely to make the same assumption that the caption writer for AP is reasonably suspected of making.