Once again, the math geniuses at OkCupid mined their users' data and struck gold, drawing some interesting connections between message/response rates and profile pics:

Now, you’re always told to look happy and make eye contact in social situations, but at least for your online dating photo, that’s just not optimal advice. For women, a smile isn’t strictly better: she actually gets the most messages by flirting directly into the camera, like the center and right-hand subjects above.

Notice that, however, that flirting away from the camera is the single worst attitude a woman can take. Certain social etiquettes apply even online: if you’re going to be making eyes at someone, it should be with the person looking at your picture.