thumbnails image loads high res only after single product page loaded

Hi,
thank you for creating woocommerce…it’s a great plugin.

We are using it here
http://www.cuchillalia.com/?s=mondador+uniblock+-1102+-kit+-lote&post_type=product

BUT as you can see initially the pics are somehow blurred …BUT if you click on the first item, go to the single product page and than return back to the previous page you’ll see the image finally is correctly loaded and you can appreciate the blurry effect is disappeared…

It could depend from woocommerce or from a plugin/theme we are using?

Thank you very much
Angelo