It's possible....
Think about it
Each pixel is an RGB value, if you keep the cam at a specific place, the pixels will change, but not much.
if you move the webcam, certain pixels change color..
so what you should do is have some kind of treshhold.
Check the image every x milliseconds, check the pixels against the older image, see if they are higher than the treshhold you set, and if so, movement was detected...
blah, decided not to post here anymore, but this just had to be rectified...