So, the trick here is to use absolute positioning calc
like this:
top: calc(50% - XYpx);
left: calc(50% - XYpx);
where XYpx is half the size of your image, in my case, the image was a square. Of course, in this now obsolete case, the image must also change its size proportionally in response to window resize to be able to remain at the center without looking out of proportion.