Interesting article found here.
4. Use a professional profile photo
First impressions matter and your cover photo sends a strong message about who you are. Make sure you have a professional looking photo to compliment your profile. Avoid any casual circumstance pictures from a party, wedding or any other group shots for that matter. And the worst thing you can do is have no cover photo at all which screams you don't care about your profile and professional appearance. And unless you really are a ninja or an actor that portrays some kind of super-hero, avoid any cosplay!