How to place sprites relative to each other

I have spent a bunch of time searching and trying different techniques but I think I am missing something very basic. I am just confused between pixels/units, screen space/world space, etc..

I have a very basic game board that is 500px x 500px and I have a prefab that is 60px x 60px (at least in my mockups and sprites I imported). What I want to do is create 4 game pieces based upon the prefab that are 20px away from each corner of the game board (see attached). What I cannot figure out is how to get the xy of the 4 corners of the game board and then add or subtract "20px" to place a piece in each corner.

Sorry for the newbie question, if there is an article or documentation that would help I am all ears.