I'm not really sure but I think that thumbs displaying a known subject (boobs are good) gets more hits. People have different favorite subjects but you can entice someone to view a pic if even it isn't their fave just by centering the subject. They'll want to get a closer look to see how you placed or handled it. I like color, contrast and movement. I don't like tiny thumbs, little red X's or non-existent thumbs (of course I still look :D)