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

Read more [here][1].


  [1]: http://devdocs.magento.com/guides/v2.0/frontend-dev-guide/themes/theme-images.html#resize-catalog-images