Cameras need light to take good pictures, and fish tanks are pretty dark. To top it off, water distorts pictures and fish move fast!
Try shining another light near the fish tank-- not directly at it or you'll see its reflection in your pictures & taking pictures of him when he is as close to the front of the tank as possible (ideally less than 2 inches from the glass).
Don't keep it there for long if your betta is shy and doesn't like it though! Some get stressed out by flashes/lots of light, and some don't care much either way. They most important thing is to keep him happy.