@pleonasm - It is just evidence, like any other evidence.
The reactions of an upset witness would be, to me, far more likely to sway someone than a grainy picture of someone doing something that might have been a crime.
And even if it is as clear as day, it is still just one piece of evidence. It is up to the defense to discredit it (if that's a possibility) and I don't think the form of the evidence matters so much as the truth of what happened.
I know that might be naive, but our courts are set up the way they are for a reason.
They aren't perfect, but are as close as we can get them in an imperfect world.