Thats going to be tricky, you can use ActiveX controls like LeadTools from www.leadtools.com to access the image data directly and perform a comparison this way. I think it would take a LOT of work to come up with algorithms that can actually say this image has changed ie. somebody walked into the frame and not been changed by photographic phenomena. I do know of technologies being used by the London Underground to monitor platform congestion and other things but this is implemented using Neural Nets and expensive technology.

I am not saying this cannot be done, its easy to check to see if an image changed. Its just no so easy to say a flickering light does not contribute to a change in state. Histogram analysis may help you to filter look for these conditions.

HTH
Neil