Research has shown this subtle distortion makes you appear less trustworthy. People flashing a smile showing some teeth were considered more competent, likeable, and influential than people with a tight-lipped smile.
Most people default to a wide-eyed look when being photographed. Grab your camera and head out into the sunset. But make sure the photo doesn't look faked. Remember, the trick is to make it look like your photos were taken while you were out and about, doing fun, interesting things that align with your interests and personality. And action shots make awesome profile pictures for guys, by the way. If you want to show off your abs, make sure the photo is taken in a natural setting like the beach, for example.