@timp: That is not theme issue, it’s your setting in WooCommerce plugin. For that go to “WooCommerce => Settings => Products => Display” and then change “Catalog Images, Single Product Image and Product Thumbnails” image sizes. See this http://docs.woothemes.com/document/using-the-appropriate-product-image-dimensions/
So for Single Product Image, you need to use image size of width at least 640, for Catalog Images, you need to use image size of width at least 560 and for Product Thumbnails, you need to use image size of width at least 300. After you change this, you need to regenerate your thumbnail using Regenerate Thumbnail http://wordpress.org/plugins/regenerate-thumbnails. So, that all your old product will have new image sizes.