Making sure all images (including those loaded async) for a single product are indexed by Google

[twilson90]

I have a product with 10 images.

On loading the product page the primary image and thumbnails are lazy loaded.

The thumbnails display below the primary image that allow the customer to view different images with a click, with a data-src attribute containing the full-sized version of the image to load.

However, a Google crawler will not intuit that the data-src attributes leads to a full sized image that it can index, so it will only index the tiny thumbnail.
Also, only the first product image is listed in the products sitemap xml.

How can I tell a Google crawler about the additional 9 images (full size versions rather than the tiny thumbnails) including their alt text, without embedding them in the page when it is loaded?