Thanks Zelda. You are indeed a guru.
Too bad, there seems to be no clear cut solution lile adding another basic extra.css rule just to do it.
I am not considering at that time to use another editor for this. I am already off limits.
Processing manually the code is also already beyond my today abilities.
When googling, I found an "old" recommendation from Kovid about it that I did not understand: to insert somewhere
<div style="text-align:center">
<img>
</div>
I tried to use it as a extra css rule but it did not work.