Using a Javascript function to add a background image on-top of a background color

I have a div that is used for a piece in a game of checkers:

<div class="blackCoin" ondblclick="makeKing(this)">

When, for example double clicked, I want to make the piece a king by adding a crown on top of the existing background color:

function makeKing(obj){
            obj.style.backgroundImage = "url('http://ift.tt/1F7udTe')";
 }

Here is the full JS Fiddle