What you need to do is something like this:
You have all your img tags in whatever way you want them to be, but with this attribute: onMouseover="swap(this.src)". Then you have your main image, something like so:
<img src="??" id="mainImg" />
Now in your head tag, you need a function, called swap (as referred to earlier):
<script language="JavaScript">
function swap(input)
{
var img = document.getElementById('mainImg');
img src = input;
}
</script>
That should do it, if it doesn't let me know...