On an eCommerce site we two images for a product.
One medium sized shown on top of the page and one large photo shown on click in an overlay.

We use http://schema.org/Product microdata on the page.

We'd like the large, initially hidden, photo to be the main image for the product, as it's the better looking one. So it's also referenced in the XML sitemap as <image:image>.

So we also put the itemprop"image" attribute on the, hidden large image.

But i'm wondering

• is it a bad idea to use a microdata attribute on a hidden style="display:none;" element?

• is there a better way to embed the main image in terms of SEO, without showing it initially?