Hello Gerand. A custom BBcode seems to work, although using the old-fashioned <center> tags </center> in HTML5 is not apparently all that recommended thesedays. It is what I have always used though. I just tested it by adding a new custom BBCode to my phpBB 3.1.10 forum under development, and it still works, although it is a bit old-fashioned, as mentioned above.
If you go to the ACP -> Posting -> BBCodes, youi will be provided with three(3) textboxes.
One(1) for the BBCode example:
Two(2) for the HTML to produce for that BBCode:
and Three(3) for the alt text to display on mouse hover etc.
Apologies for the Sesame Street English, but that is how HTML likes it, and so the BBCode should conform to that lazy standard. It's only code, anyway. No one's going to read it.
I believe that this has worked just fine for images over all of my past forums when I have bothered to add any custom BBCode, but haven't bothered to test it just now, because I trust that anything inside the <center> tags is generally centred on the page.
I hope it helps.