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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.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