There are a few steps to make this work.
First, you have to get images into LabVIEW from the webcam. Webcams don't present their data in a standard format, so there's no guarantee that anything will work, but there are a couple of tools out there. NI provides supported tools for GigE and other expensive types of cameras. For USB, NI provides NI-IMAQ for USB but support is limited and your device has to work with DirectShow.
http://zone.ni.com/devzone/cda/epd/p/id/5030
If this doesn't work, you'lkl have to try some other tools. Search for "LabVIEW webcam" and a few things come up, like:
http://mindtrove.info/software/labview-webcam-library/
If it doesn't work, it's because your USB camera doesn't conform to one of the "standards."
Once you have the image in LabVIEW, you need to find a method to look for differences in the images. You can do this with the array tools in LabVIEW, but you'll be starting from scratch. A better method is using the Vision Module:
http://sine.ni.com/nips/cds/view/p/lang/en/nid/2881
These threads show some ways to use the Vision Module to do motion tracking:
http://forums.ni.com/ni/board/message?board.id=170&message.id=40536&requireLogin=False
http://forums.ni.com/ni/board/message?board.id=200&message.id=14307&requireLogin=False
Hope that helps!