If you have any doubts as to the effectiveness of subliminal messaging then you haven't spent much time in the advertising community. KFC understand the way peoples minds work. They have a picture of money that you have to magnify to see. They know that people have a positive associations to money and hence they capitalize on it. They could have made it obvious and just had money instead of the lettuce but you would probably would have a rather negative emotion because you realize how odd it is. Instead they send a message directly to the subconscious mind. They want you transfer all those positive feeling you have towards money to there sandwich.