Deleting thumb doesn't delete the entries in database. The reason why you are required to delete thumb is just a sort of "clear cache", because the thumb generated is essentially some kind of "cache files".
Here's how it works:
1) There's an original full size image for ever image you upload.
2) Any time there's a request for smaller size images, nopCommerce generates a thumbnail from the full size image. And this newly generated images is stored in thumb folder as cache files, so that re-generation is not required for subsequent requests of the image of the same size.
So to totally refresh the image, you have to do 2 things:
1) Remove the entries in database. This can be done manually (inefficient) or via a quick SQL script.
2) Remove the thumbnails.
Hope this explains? :)