They are "serious" pictures, typically taken from the front and looking very professional. They are wearing at least a shirt, avoiding hoodies or similar casual clothes. They transmit "professionalism".

I lack this. It's not that I cannot wear shirts or similar clothing; it's just that I don't want to transmit "professionalism" merely through my profile picture. The problem I see is that if your profile/CV has a less-than-adequate picture, it's likely it will be rejected.

This probably doesn't apply to FAANG companies (the best counterexample is probably Peter Norvig and his hawaiian shirts: https://en.wikipedia.org/wiki/Peter_Norvig)