Here's my list:
Making Things Better

The first two are easy to fix once we realise that a player should only be able to click on a card back.
So all we need is something like:

  function card_clicked(image) {
    if (image.src=='cards/back.png') {

You may like to try this and then come back here to find out why (surprisingly) it doesn't work!

Try this to see what the problem is:

  function card_clicked(image) {
    alert(image.src);

So we have to figure out a way to use the browser's way of storing an image's source.