Question

This js fiddle runs with no errors or problems:

http://jsfiddle.net/3j45B/2/

However, when embedded within http://www.zabb.co.uk/untitled6.html it does not work? Any suggestions please?

---

EDIT:

Further to your comments and suggestions below a solution has been found, thank you all:

$(document).ready(function() {

    $('.thumbnail2').click(function() {
        src = $(this).attr('src');
        if (src != $('.img').attr('src')) {
            $('.img').fadeOut(250, function() {
                $(this).attr('src', src).fadeIn(250);
            });
        }
    });

});​

Important bit to ensure event works is to wrap your jQuery code within:

$(document).ready(function() {
    // ...
});​

See following link for further information regarding this function:
Introducing $(document).ready()

Answer 1

Wrap your embedded script in a $(...) or $(document).ready(...)

jsfiddle did that for you (see the dropdown box on the left that says “onLoad”?).