When you add image to product in back office, for example, Prestashop generates all necessary types of images in Preferences > Images. Like 123-home_default.jpg, 123-large_default.jpg.. and so on.
You could change those generated images manually, for example with JPGMini app/website to reduce size of images and upload new images,
And then if you click on regenerate, it will delete those "Manually uploaded thumbnails" so that is warring about.