Nipples are not a sexual organ. But for sure someone feels good when his (or her) partner takes care of this part of his (her) chest. Also, when you feel horny the nipples usually are getting hard. In a relationship the nipples are rubbed, or licked, they could be sucked and even bitten … a great help to reach the climax of a sexual intercourse.
So for sure the guys of these selection of images are just waiting that something happen to their nipples by exposing them in that way… so just imagine what you can do to make them happy!