If you use an obvious hack (killstreak hacks, infinite ammo hack, etc.) then a mid kid will report you. The game-masters will simply look at the "screenshots" you speak of.
I believe that they won't waste their time looking at EVERY screenshot taken (that would take years). I think they only look at the screenshots of people who have been reported frequently.
It doesn't matter if they take a screenshot once a second or once a day, you will be caught sooner or later by a screenshot if you hack. I highly doubt someone who has never been reported or caught by VAC has ever been banned, even if he hacks 24/7.