I sometimes smile. I usually say "cheese" or whatever when asked though.
Accept the past. Live for the present. Look forward to the future. Robot Fusion
"As our island of knowledge grows, so does the shore of our ignorance." John Wheeler
"[A] scientist looking at nonscientific problems is just as dumb as the next guy." Richard Feynman
"[P]etabytes of  data is not the same thing as understanding emergent mechanisms and structures." Jim Crutchfield
I'm ENFP my brother is IN_P. He does NOT smile in pics. I took a photography class and bugged him once b/c I needed a subject. The best he does is a 3/4 cocking of the head "Captain Morgan" style and looks bemused. He'll wrinkle his brow and purse his lips and look deep in thought or else slightly self-conscious. He does NOT smile usually, when he does, it's a small curling of the lips with no teeth.
As for me, I HATE how I look in pictures. And if i don't smile, it's x3. I used to have a funky overbite whcih I felt self-conscious about but closing my mouth made my fugliness more apparent. And I hate looking fugly.
I will make my biggest often borderline psychotic cheesy grin and freeze my face to comedic effect. My brother says I also open my eyes really wide and "look psycho". Sometimes he'll yell at me to stop if he's taking a picture.
The purpose of me looking batty with my crazy smile is so people will KNOW I am NOT trying to be serious and tell them "yes, I know I look fugly now leave me alone" and not use my picture. It's the equivalent almost of flipping off the camera, because doing so makes the shot unpublishable/unuseable in most cases.
If I have to look fugly, I might as well be charming about it.
I almost always smile in pictures, though I NEVER say "cheese"- even if told to do so (or especially if told to do so!). I've had the same smile in pictures since I was a baby, and it's generally an amazingly cheesy smile!
My isfj mom and esfp sister always smile- my intp dad tries to hide from the camera- I think the only pictures in which he smiles he's with my mom.