You are right that the raw sensor data should always be preserved. But sticking with the license plate example, you could challenge a picture of a single car with a visible license plate far away in a wooded area, but it would be hard to refute a picture of the same car in a parking lot surrounded by other (non-suspect) vehicles whose presence there at the same time could be independently verified. In other words, if I can show that it accurately read the license plates of 9 other cars, the chances that it got yours wrong go way down.

That's assuming a single photo taken in the dark by an investigator. With a fixed security camera you would have an even larger basis of comparison, with a population of hundreds or thousands of license plates against which to rate it. I predict that before long we'll see preemptive certification for devices warranting the reliability of their image pipeline out to a certain distance at either the manufacturing or installation stage.