Your profile picture is how the online world perceives you, so make it a good one. The folks at Photo Feeler surveyed 60,000 ratings of 800 profile photos and came up with a few rules to look good, including "squinching" your eyes and smiling with your teeth showing.
The gist of the concept is this: wide open eyes commonly denote fear, whereas slightly squinted eyes portray comfort and confidence. As a consequence, squinching eyes garner an average gain of +0.33 for Competence, +0.22 for Likability, and +0.37 for Influence.
The research found other useful patterns in making a good profile photo. For instance, a shot of your bust (head and shoulders) or torso (head to waist) score high, while face-only shots or full-body photos are not preferable. Photo Feeler put all the information in this helpful infographic, but you should check out the link for an explanation of each aspect.
New Research Study Breaks Down "The Perfect Profile Photo" [Photo Feeler via The Daily Muse]