Add prices to allow visitors to purchase work.  Or, you may wish to only display prices to private visitors.

Turn on the price field

Login > image gallery (with the images) > Image fields > Price, set this to Public or Private and set it to display on the images.

Display Buy button by turning on the For sale option as well.
How do I password protect prices?
Change the price visibility from Public to Private, and then create private user name and passwords to give out.  These private users can login at