1. It is not optional. Weird ideas fill a void. If you don't have a photo, then viewers of your profile will use their imaginations to create their own image of you. Leaving people to their imaginations usually is not in your favor. You will inevitably disappoint in person.
2. About disappointing in person, use a realistic image of you that is relatively current. Over ten years? Hair dyed a new color or fell out? Gained or lost 40 pounds? It is time for a new photo as you will most definitely shock in an in-person meeting. This is definitely a "what you see is what you get" situation.
3. Dress appropriately for the audience and the business demographic you are targeting. If you are looking for business or a job in tech then dressing in a formal dark suit and red silk tie might be off-putting when the business dress code for the company is yoga pants, jeans, and hemp shirts. This is probably the most important tip as to how you are dressed creates a first impression that is hard to erase. Plus, take off your tint-adjusting glasses when outdoors.
4. The setting and pose is a question of relevance. If you are in law enforcement, outdoor sports, or any risky business then a skydiving shot of you at 9,000' might be a great pose . In chartered accounting, banking, brain surgery, maybe not so much. Same could be said for the really cute pose taken at Burning Man or Coachella. Keep your personal life off Linkedin.
5. Look forward at the viewer or towards the center of the screen. This is crucially important now that Linkedin has moved photos to the left side of the screen as you view it. If your body and head are facing out to the left as well, the portrait image creates an unconscious psychological dissonance with the rest of the layout.
6. In the same vein, the background color and view can add or detract. An all white background ranks higher than the ubiquitous corporate photo gray or worse Hollywood black. White is young, edgy and won't fight with your Linkedin background banner that hopefully, you have customized (a topic of another article).
7. Finally, look happy...not cool, inviting, engaging, business-like, professional, or a leader (whatever that is), Just be happy when the shutter snaps. Happiness is contagious and welcoming to everyone.
Trust in the portrait-making process,and hire a professional photographer, not your wife or partner. Pick a decent background and outfit. Don't worry, be happy..