Hi,
Yes, it's possible on the same website.
Normally, you do it like that:
- you upload a small size image in the "image" section of the product so that the customer can see a small version of the image on the website frontend.
- you upload a big size image in the "file" section of the product so that the customer can download that image after the purchase of the product.
Regarding the watermark, if you configure one, it would be added only to the small image you upload, but it will be for these product and the other products of the shop. So that might not be a good idea. Note also that the watermark is only added when you upload the image via the product edition interface. So for the products you already have, they won't get the watermark added. So it might be a viable solution to use the watermark taking this fact into account.