Image sensors are essentially advanced motion detector, armed with a camera. If an intruder crosses in front of an image sensor, it will trigger the alarm the same way as a motion detector, but will also snap pictures of the intruder and send them right to your smartphone and/or online account.
This technology is very helpful to confirm break-ins and identify any intruders.
Additionally, with a smartphone you can access your image sensor and take a picture to see what is going on in your house at any time.
You can do this anywhere you have internet access, anywhere in the world!