Where should THUMB be during shakehand grip?

Baal wrote: Yep, nearly everybody (elite players also) slides their grip around at least a little bit from shot to shot, including where their thumb sits.  Also, different players can differ a lot in where their thumb sits most of the time.  That is clear from the photos posted by the OP.  This is one of those things, like how much the elbow is bent on a forehand loop, that doesn't have just one correct answer.  Also, some players change their grip more than others as they hit different shots.  I have even known some elite players who say they don't change their grip but when you watch them closely you can see they are doing it some.

This is true, but I think it is important to note that the elite player perceives that they aren't changing their grip as they play. Their perception of what is going on might not be reality, but based on their viewpoint it is. So when you are hitting and trying to analyze yourself, you should perceive that you are not changing your grip either. If you're trying to have a stable grip that is...

It could be just me but I move a bit both my thumb and my index finger depending on the stroke.

1) serve - both are holding the racket a bit lower (off the face) than usual so I could do a faster swing/snap motion

2) opening FH loop close to body - both fingers are on the face, deeper into the racket so I basically perform the loop as if the racket were my palm

3) counter-loop or kill-loop - index finger a bit lower but still on the face, thumb slides down to the top of the handle, so the arc of the swing is a tad longer

4) FH block (simple and fast) - index finger moves up the face becoming less horizontal, thumb stays in place

5) FH block (slowing the ball) - thumb goes lower, index finger presses on the face forcing the ball down a bit more

Etc.

Some players say that they never change the grip or finger position but I really really doubt that - that's what they were taught but literally nobody can do that. Some elite players also say that they change fingers' position for a smash or a BH loop... but generally they do indeed prefer not to change it much because it creates problems with transition from one shot to the next one.

OP, also note that the photos were taken during the rally. The fingers and thumb move around as the player adjusts his grip for whatever stroke he's playing.

Thumb should be resting on the bevel on the handle. You should feel yourself pinching the paddle with your index and thumb.