We should run the catalog images command: `php bin/magento catalog:images:resize`.

> Situations where this could be necessary might be:
> 
> - After you import products, which might have images of various sizes <br/>
> - If images were resized or deleted manually from cache

> Each image assigned to a product must be resized in accordance with image metadata defined in a module’s `view.xml` configuration file. After resizing an image, its resized copy is stored in the cache (`/pub/media/catalog/product/cache` directory). Magento serves storefront images from cache.

Source and more information here: http://devdocs.magento.com/guides/v2.0/frontend-dev-guide/themes/theme-images.html#resize-catalog-images