I disabled product thumbnail generation in the import configuration. How can I generate them after importing products?

If thumbnail generation was disabled in the import configuration, generate them after import through the PrestaShop back office menu.
To do this:
1. go to the tab:
> in PrestaShop 1.6 "preferences -> images":


> in PrestaShop 1.7 "design -> images":


2. then at the very bottom, in the "Regenerate thumbnails" section, select the product thumbnails and disable removing previous thumbnails, since we want to generate thumbnails for products that don't have them. If we wanted to regenerate thumbnails for all products, leave this option enabled, and if the process takes too long and stops, we need to go back to this tab, disable this option and restart generation, refreshing the page after each interruption until the thumbnail generation process finishes (when the removal option is disabled, refreshing the page after the process stops continues it):