Motion detection with a camera is not as simple as it might seem. It sounds easy to compare two images to see if they're different but lighting can change between the samples meaning that the samples are very different even though nothing has moved. Some inspection systems use cameras like that and they're jolly expensive.

I think I'd be looking at something simpler such as infra red motion detectors. These have a digital line that goes high if it detects something. So I'd use IR to detect movement and then take a snapshot with the camera just to keep as evidence. Some cameras have an IR sensor allowing the software to start recording if the IR detects motion.