My teeth are not stark white but look nice but when I have a photo taken with a flash, my teeth do not shine and look darkish in the photo. Any ideas?

Why are you focusing on your teeth so much? What about the rest of the picture?

If your concerned that your teeth are stained the dentist will be able to help you out.

It could be anything from you needing to have a whitening treatment, change your lipstick color (assuming you wear lipstick) or simply bad lighting which creates weird shadows. It can also be the camera, if the settings aren't right.

I'd start with a whitening treatment - you can try Crest White Strips or something similar, or even look for a dentist which offers in-office treatments such as Zoom.