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
- 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.xmlconfiguration file. After resizing an image, its resized copy is stored in the cache (
/pub/media/catalog/product/cachedirectory). 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