More importantly, what would have happened if a car (that was not speeding) just happened to be visible in the photo?

I’ve never received an automated speeding violation, but have received turn-on-red ones before. In those cases, the violation includes a link where you can see a video of the whole scenario and offers a button to click where you claim it was not actually a violation. Hopefully the speed cameras also have such a system where you have some reference points to calculate time travelled between frames in the video (with a know frame rate).