// option choices image swap function
function OptionSelectionChanged(dropDownId) {
    var optDropDown = document.getElementById(dropDownId);
    if (optDropDown != undefined) {
        var opt_index = optDropDown.selectedIndex;
        var optid = optDropDown.options[opt_index].value;
        var images = eval(dropDownId + '_Images');
        var pImage = document.getElementById('miImage');

        // determine the resized and large image url's and update
        if (pImage != undefined) {
            if ((images != undefined) && (images[optid] != undefined)) pImage.src = images[optid];
                // image will be force updated to the default image when swatch image is absent
            else if (_DefaultImageUrl != undefined) pImage.src = _DefaultImageUrl;

            // update the large image url used by fancybox, so it show correct image for selected option choice
            var pathRegexp = /.*GetImage\.ashx\?Path=(~|%7e)(.*?)&.*/im;
            match = pathRegexp.exec(pImage.src);
            if (match != null) {
                var pImageUrl = document.getElementById('ProductImageUrl');
                if (pImageUrl != undefined) {
                    pImageUrl.href = match[2];
                }
            }
        }
    }
    return true;
}

<script>
var _DefaultImageUrl;
</script>