"Stealthy" pictures on iOS devices without jailbreaking:
Open the camera app, then immediately pull down the notifications from the top (as shown below). This covers the camera viewfinder while leaving a mostly hidden ghost image in the background.
Use the Vol+ key to snap a pic. Note: sometimes, the background image will move as you move your phone. Other times, the background image doesn't move, but the camera will snap where it is pointed at the time you hit Vol+. That may seem obvious to some folks, but not so to others. Experiment.
Handy for taking photos where the viewfinder might serve as a distraction (plays, football games, concerts, etc). Could also prove handy in settings where photography is frowned upon, but you use that at your own risk (museums, etc).