I had my teeth whitened, and they are already yellow again. Why is this happening, and how can I stop it? (Photo)

The initial photo after whitening appears that the teeth are dried out which make them appear whiter. When they rehydrate as they should, they do look alittle darker or more yellow. So the first picture is deceptive. The final photo does look as though you did get some good whitening effect. Now you need to followup with in-home whitening in a custom tray to continue to get results to whiten where you want. Ask your dentist about this