Multistore Images are not coming after Upgrade 2.2.6 to 2.3

You should try using the image resize command to pre-generate all necessary resizes.

php bin/magento catalog:image:resize

This command gets all the images sizes that have been defined in the theme XML and regenerates the images in their correct folders. after that please remove image cache and flush Magento cache

You can also check the command documentation for more information http://devdocs.magento.com/guides/v2.1/frontend-dev-guide/themes/theme-images.html