"Knowing what to do" and being able to do it well are different. I have had good photographers take pictures of me. They have used the appropriate lighting and angle and helpfully tried to coach me in what to do--how to pose, how to smile, what to wear. And indeed, their pictures turn out better than most snapshots. That doesn't mean that I am able to use their advice effectively--to hold an unforced smile and keep my eyes open at the same time, avoid tilting my head funny, or not look frustrated and impatient after the 50th shot. It's a difficult skill for me, and while I expect I could be better with practice, it's not high on my list of desired skills to improve. So I'm not photogenic. Which means merely that I don't have these skills now and don't pick up on them quickly, but when you look at my awful pictures it's no different than if photogenic-ness were some immutable inherent quality.

(But I'm off the market anyway. On the upside, my partner was pleasantly surprised when he first met me in person that I was better-looking than my photos suggested.)