It honestly looks pretty good to me, but if you really want to go for another method, you can swap your iframes for divs (if I remember correctly, div has a style option to handle overflows by adding a scroll bar), and use JavaScript to edit the img tag's src attribute whenever you click a preview image. To do that, you would use a block of code something like this either embedded in the page or as an external document:
Code:
function showImage(image) {
document.getElemenyById(imageID).src=image;
}
... and then add this to your preview image tags:
Code:
onclick="showImage('URL')"
... and add this to the full-size image tag:
... and that should do it. Just replace "URL" with the image's location (keep the single quotes around the URL though), and get rid of your <a> tags... though, if you still want the user's mouse pointer to change to the pointing finger, you can just swap your current <a> tags for <a href="#">.