If a new detailed image is uploaded for a product in the administration panel and its thumbnail is not displayed on the storefront or a thumbnail of a previous detailed image is still displayed there, it can occur because automatically created thumbnails were not re-generated after uploading a new image.
In this case you need to clear the thumbnail cache in order to re-generate the automatically created thumbnails. In order to do it, in the administration panel go to Administration > Storage and choose the Clean up generated thumbnails tab