Bulk Delete Product Pictures

The scenario is I found/produced better pictures than the ones I already had upload/assigned to a range of products. Obviously, I'd like to replace the old photos with the new but NopCommerce seems stubbornly attached to the original ones. What am I missing? I delete them from Thumbs folder and I try uploading Excel file with blank path but it still is generating the old thumbs.

Also, how can I define 4th and 5th photos etc. I see you can only upload more than 3 in Admin and not using and Excel.