If You Have A Neutral Expression, It Means...
The most recent research on profile pictures found something very specific: if you have a profile picture where you're making a neutral expression, have short hair, black hair, and you're wearing a hat and/or sunglasses, people won't see you as very friendly. They'll see you as more stand-offish and less approachable. This seems pretty obvious: if you're not smiling, you're going to be seen as less friendly. What is interesting is the short, black hair findings.