There is now an option to upload print versions of your photos in the cog wheel in the toolbar below photos in full size. The print versions will not be displayed online, they will only be used for prints. You can upload in TIFF or JPG format up to 200 mb.
Even as a free member you can sell prints of all photos on your profile, so make sure to mark your photos as avilable for sale in Account Settings so you don't miss out on any sales, it just requires a few clicks.
If you really want to have the chance to sell a lot of prints, activate "Third party print sales" by signing the agreement as well as print sales by Eurographics, the biggest prints provider in Europe with stores in 60 countries which your photos might get selected for. Make sure that you have uploaded high resolution print versions for all photos you make available for sale.
Upload a print version in the cog wheel in the toolbar below your photos in full size.