The New Bits
  var card1='';

This is where we store the first card clicked.
Notice that it's outside the functions and so is not lost when the function finishes whereas variables inside a function are erased when the function finishes.


    if (card1=='') {
      card1=image;

In English: if this is the first click, remember the card.
Note the double equals ( == ).
The overall pattern is always:
if (condition) {do something} else {do something else}


      n1=n1%13;

% 13  means divide by 13 and deliver the remainder eg 28%13=2


      card1='';

Making sure the next click is seen as a first click.