Not sure how long this has been part of iOS, but I just stumbled on this yesterday. A fantastic tip.

Go into Messages and tap on a recipient. Now press and hold on the camera icon (to the left of the text entry field). Don’t lift your finger. After a brief pause, a quarter circle control will pop up under your finger with an x, a camera icon, and a video record icon. Your camera will become active at the same time.

If you lift your finger, the control will stay in place and you can cancel (tap the x), or compose your picture, then tap the camera (for a still image) or the record button (for a video).

Alternatively, you can keep your finger down and slide it over the control you want. For example, press and hold to bring up the control and the camera, then slide over the camera icon and lift your finger to take the picture.

The only downside here is that once you take a picture, it will immediately be sent. I do love the convenience, but I wish there was a gesture that would let me retry if the picture wasn’t quite in focus.