Besides needing a great face, hot body and big dick I always go for big nipples -- the area around the actual nipple and the nipple itself. Bonus points if they look swollen and worked over. But not extreme (although there are a few pictures that go near the line).
In addition, I usually select pics that are color, include the face, and are of regular/masculine guys. Recently I've seen some nice vids with hot nipple guys that I've added to the site, but most of what you'll see are pics and a few gifs.