CameraButtons lets you use the volume buttons to take pictures but not only that, you can configure it so that pressing the Volume + button to take a picture (or video) regardless of your iPhone at the moment is in the photo function or video, will be changed. It also lets you enable HDR with the volume buttons and switch between the front and rear camera.
Likewise, you can activate the camera from the screen leaving lock down button - volume.
Do not add icon, you can configure the settings of the iPhone.
You can download it for $ 0.99 in Cydia.
You need to have done the Jailbreak.