The blog of the dating site OkCupid is crazy compelling. In this recent post they bring some serious statistical analysis to bear on the effectiveness of profile pictures. Here are two charts showing what kind of face you should be making in your profile picture to generate maximum interest from the opposite sex.Men should definitely not make eye contact unless they're making the lip-pouting "flirty face."
But for women, do make eye contact, and make sure to smile.
That's only the tip of the iceberg. The post goes on to investigate the effect of photo context (women should not "pose with an animal"; guys should "show off muscles"), wardrobe choice (guys: dress down), and more. Just adding to the store of human knowledge.