You can't directly as otherwise your server would have to generate all sized images and thumbs everytime anyone views your site. Your site would become unusable if you have more than a few visitors at a time.
But if you upload images in the same size or smaller than needed on the image page for the sized images, Zenphoto will not process them and use the full image instead. Of coures you should not use the watermark option, too.
Btw, who is "NG"???