It is known that they do take screenshots, but it's not every kill, as said above it'd take years for them to process all images. They probably take 1~2 screenshots of you per match, if not per day... And the screen shots are not "manually reviewed" by them, that's just bullshit. They have some imaging recognizing algorithm to detect stuff that shouldn't be there (aka some "weird" red pixels creating some boxes all over the screen lol).
What they probably do is (at least would be a good strategy IMO) to have a image queue. All screenshots taken are placed in the end of the queue, those analyzed by the algorithm that got detected with weird things on it are placed on the start of the queue, and their anti cheat team look at them to see if they really have something that is not allowed, then BAM! those that that algorithm said was clear never got analyzed ^^ Faster and almost flawless (I'm shit at explaining stuff... hope you guys understand what I meant)