It took me a while to realise that the player's first click must be on a back because, at the start, that's all there is!
So I set up a variable like this:

  var back_src='';

... and used the fact that it is empty at the start to recognise the first click - like this:

  function card_clicked(image) {
    if (back_src=='') {
      back_src=image.src;
    }

So here's how I changed my program:

  var back_src='';
  function card_clicked(image) {
    if (back_src=='') {
      back_src=image.src;
    }
    if (image.src==back_src) {

      ...

    }
  }