document.observe("dom:loaded", function() {
    $$('img.thumbnail').invoke('observe', 'click', function(event) {
        var thumbnailSrc = this.src;

        var previewSrc = thumbnailSrc.replace(/_t\d+\./, '_preview.');
        var previewElement = new Element('div', {'class':'preview'});
        previewElement.observe('click', function(event) {
            this.remove();
        });
        previewElement.setStyle({
            'position': 'absolute',
            'cursor':'pointer',
            'zIndex': '1'
        });
        previewElement.update("<img src='" + previewSrc + "' />");

        Element.insert(this, {before:previewElement});
    });
    $$('img.thumbnail').invoke('setStyle', {'cursor':'pointer'});
});