Thumbnails and previews are generated automatically when you upload a picture. You don't have to do anything except upload the image. As to deleting the image... geez, I've been looking around and guess what? I can't find an answer. It's hard to believe the developers of the image module didn't take that into account. I'll do some research and get back to you as soon as I have an answer. If in the mean time, you find a way, please let me know.