Lets say you have a image
and the dimensions are
Width(X) 300px X Height(Y) 274.3px
the page size is 375 X 375
This is very simple
(375-300)/2
374-300=75 & 75/2=37.5
So that is your X dimension
Now (375-274.3)/2
375-274.3=100.7/2=50.35
that is your Y dimension
Very simple
So X 37.5 & Y 50.35
Done