The image will be taken when the aircraft lands while still under automated control. I don't really know what will happen if you are manually landing it. It very well might not be taken. The idea behind the automated landing is that the pilot can adjust the flight as desired to land safely while still being able to receive automated flight event messages from the aircraft.
You can see that the image is taken when the image count is incremented by one at the time of landing.
We take it at the end because the barometer tends to drift as the aircraft warms up and capturing it at the end gives us the takeoff location and the drift in one shot. If you are concerned about it you can certainly take your own ground reference image by tapping the top half of the camera preview area with two fingers before taking off.