Thumbnails regeneration you suppose to do every time you upload images and change image sizes.
- Log into your PrestaShop admin panel and go to Preferences -> Images.
- Find Regenerate thumbnails button there.
- From the drop-down menu select a type of images that should be regenerated – product images, categories images, etc.
- Click Regenerate thumbnails button. All the thumbnails will be re-built for you.
Hopefully, this tutorial was helpful for you.