In order to allow access to the original images and printing capabilities to selected users only, we'd recommend you to set small Popup Viewer Dimensions when uploading your images and combine this with Access Restriction for the original images.
This way, the larger view of images, which appears when you click on the thumbnail will be with too small resolution to print. Only users who are given the password and whose email is added to the list of authorized persons, will be able to access the originals and make quality photo prints.
We can also recommend you to create a page which explains that the images are copyrighted and that access to quality resolution and prints can be obtained after filling in a form. If you wish to sell the right to use the images, add a PayPal button as well. Once you receive a PayPal notification for received payment, you can add the user's email to the list of persons authorized to access and print the originals and send them the password.