I've been on holiday.

Surely the validation errors are obvious and self-explanatory? It seems you've forgotten the closing tag for container2. And the other errors are because you've put Javascript directly in the <head>. Put it in a separate JS file, preferably in one of the ones you already have. I would also recommend putting ALL your JS in a single file. Making the browser request several different files puts more strain on the server and makes the page load slower.

You need to learn to debug javascript by yourself. I can't tell why this isn't working immediately just from looking at it. rolloverInit() must be being called, but the mouseover event appears to not be triggered. One of the most useful things in JS debugging is the alert() function. For example, to check that the onmouseover event is being triggered:

Code Javascript:
      thumb.onmouseover = function() {
        alert('moused over');
        var myImage = document.createElement('img');
        myImage.setAttribute('src', bigsrc);
        myImage.setAttribute('alt', this.alt);
        // etc.
So if you see the alert, add one a bit later on, preferably when something should be happening, to work out where the code is failing.

Also, you can get rid of the ID on that thumb, since the class attribute is what we are using.