I am a small hand beginner guitar player. If I play on fret 7, my thumb twisted left a little. Is it wrong? (Sorry for the silly question)
In the beginning, try to keep your thumb in the center on the back of the neck. This will be difficult to do as you go higher (further downward) on the fretboard. However, this position provides the optimum amount of leverage required from the thumb to make sure you do not have to press to hard.
That said, after some years of playing, I do find my thumb flopping all over the place. And when I see that my thumb is hanging over the top portion of the fretboard (low strings), I do notice that I tend to press the strings much harder.
Just keep at it and understand that hand size and finger length have nothing to do with skill.
You'll find in time that you sometimes won't even need your thumb on the back of the neck - anywhere.For barre chords, which you may have started, it's good, especially at the beginning, to have the thumb centrally at the back, but I think your concern is more about where on the thumb you press. Firstly, don't press too hard! Secondly, it doesn't matter which position, rotated or not. The other factor is that when you push the guitar to your left, the thumb will flatten out anyway.
It is not wrong at all. I too have very small hands and I often must rotate my thumb in the manner you pictured in order to play certain chords, particularly as I get higher up the neck.
If every guitarist had the same size hands and same length fingers, they could all use the same thumb position. But different hand anatomies will usually necessitate different hand/finger/fretboard geometry. What I mean by that is that a guitarist with extremely large hands (the meat of the hand) and long fingers will need to orient his thumb and fingers differently as they relate to the fret board than someone with average hands and small fingers or small hands.
Even the thickness of the hands can affect the position the hand might need to assume to get the fingers in the correct place to play certain fingerings of particular chords.
To the extent you can, try to maintain the thumb in the center of the back of the neck and if possible keep it flat against the neck. But if you need to rotate your thumb to get your fingers in proper position to cleanly fret the strings for a given chord, don't hesitate to make the adjustment.
Keeping the thumb in a consistent orientation to the back of the neck is more important when playing classical guitar where your fingers need to move around to play individual melody notes while keeping the thumb in the same place for stability and a consistent reference point.
But when you are playing chords (all notes fingered simultaneously) and alternating from one chord to the next, your thumb will most likely shift position with each new chord.
In the picture you appear to be playing a barre chord and that is exactly how I would play the same chord. If I try to flatten may thumb in that situation, I can't properly place my fingers.
No worries you are just fine!