How do I detect intruders with CameraGUI?

I have to create a program which has to be able to detect intruders by using CameraGUI for Matlab in its RealImageProcessing mode. I think I can detect the intruders by subtracting the last "picture" to the previous one. However, I don't know how to extract the pictures from the code of the CameraGUI algorithms. I know that in each interaction, just the last picture is saved, but I need the last and the previous one. Any idea? Could anyone help me?

Thanks a lot.