Plus why did it have to be a stock photo? These were done by the publisher who should be customizing book covers for their artists a bit more (I know, only the bestselling authors really get full publisher attention) and if just using a photo it's not that hard to take a new one. Particularly a back of the head shot lots of regular folk or models would pose.
There are photos of African people, even on the free sites like Pixabay, although not many. I make covers for the wattpad writers, so I know. Stock photo sites have dozens of images for every race and situation, and the prices are not too steep for multi-million-dollars publishing giants. Also, cover artists should be able to paint a cover too. What they did with her cover was inexcusable: just trying for the cheapest option.
I really ought to put myself out there somehow as a cover artist...