Hello again..;-) It is sort of possible as the full iamges are left untouched. Zenphoto is not a shop and there are no shop plugins either to do what you wish to do out of the box.
Also the place where full images are stored is generally known unless you change a few things.
So you will in any case have to do some modifications to your theme code or write a plugin to achieve this download functionality as that surely should be tied to a purchaser specifially.
I suggest to install Zenphoto and play and look a little around how it all works.
Also look at the troubleshooting how the image setup works (There is a diagram). Zenphoto generally does not touch the full images uploaded so there is by default no watermark on these. Note if you are planning to sell fine data images in high resolutions your server needs to have the power to use them, as Zenphoto creates all other images from these.