The limits are 400x400 to 7680x4320 pixels, under 8MB, PNG or JPG. The more useful part is what LinkedIn does to your photo after you upload it.
Profile photos.
What a hiring page, a LinkedIn profile or a company directory actually needs from your picture.
A headshot has three jobs: be recognisably you, match the room you are walking into, and survive being shrunk to 48 pixels. Everything else is decoration.
Almost always one of four things, and only one of them is about the original photo being soft. The other three you can fix in a couple of minutes.
Most places show you a circle or a square. A few want a portrait rectangle. Shoot once with room around you and you can produce all of them from the same frame.
Video call avatars are shown small, cropped to a circle, and often scaled twice. Three habits fix almost all of it, and none of them involve a better camera.
The photo goes on a website, a printed programme, a holding slide and a social card, each a different shape. Sending one small square causes most of the problems.
The question worth asking is not whether software touched the photo. It is whether someone who meets you would recognise you from it.
You upload one square and get a circle here, a wide banner there, and your chin missing on the third. The fix is to build the photo for the worst crop rather than the best one.