Each of the photos that you find in any gallery has a link to select purchase options. There is an obvious shopping cart icon at the bottom of any thumbnails or the even more obvious green "Buy" box at the bottom of any image that is clicked on to open.
There will be multiple options available for product types with multiple options within each category . It is pretty easy to select the desired product using the overlay to help in choosing size, dimension and crop.
There is a small information button on each of the product names that shows you the print material, image mounting details and wall mounting specifics.
If you have any questions, please contact me at firstname.lastname@example.org.