I think I have accidentally discovered a way to cheat at classic cash, so being the honest not-cheating type, I am reporting it for a possible fix.
I have noticed that when I load the page, most of the time one of the posters flashes onto the screen faster than the rest. Since noticing this and selecting that one, I have won every time I've selected the early poster. When one doesn't stand out as being there first, my chances are much lower, with hardly any wins.
So it's possible that at certain connection speeds/load times, the winning poster loads first and thus gives itself away.
I like having the game option so I hope this doesn't make you take it away, and it's possible that it's coincidence, but I've just won for the fifth time using this so I figured it only fair to report it as a possible bug.