There is an article on CodeProject that explains how you can get the difference between two images. While you're having a look at the camera screen, you're watching images being changed as frames on the screen. Each of them in real are frames being provided to you.
So, each frame might differ to the previous one in many ways.
Simple image comparison in .NET[
^] explains how you can get the difference.
In that article, he explains that on the byte level (pixel level) each pixel has a slight difference that human eye can never even think about noticing. So you might not detect any change but the computer would find a change in the new image and will "Click! Click! Click!" in 3rd part of the second and so.
Read the article above, and code it out for you. All you need to do is, to make sure, that the image difference is being handled. After that you can get the pixels from the screen and saved as a screenshot. I have written a tip for that,
Saving a Screenshot Using C#, A.K.A "Console Monitor"[
^], here you go. Read these both links, and I hope you will get what you want to get.