Not very many people know that the volume buttons on their iPhones work to take pictures, but this is good to know in case you are unable to tap the shutter for any reason. You can also take photos on your iPhone in burst mode, which will take ten photos per second.
To take one picture on your iPhone, just press your volume button once. Then, if you want to take photos on your iPhone in burst mode, press and hold the volume button down. You will hear a rapid shooting sound to let you know that the pictures are being taken. If you are wearing headphones with a volume control, you can use that volume control to take one photo at a time. However headphone volume buttons will not work for burst mode.