Upload & Sell: On
I would say that taking an approach that targets interior designers is the way to go if you are trying to sell fine art photography. If you Google this topic you will find a number of blogs that give tips for using Pinterest to sell or market photography services. As I understand it, adding value to your Pinterest page is recommended -- such as pinning to other products and services that your followers might like (in your case, if you want to market to interior designers, it could be interesting architectural spaces or pieces that might complement the style of your work). Pinterest will often pay off by increasing trust and loyalty among your customers. You might not be able to quantify its numerical value but you can use the service to create more valuable connections with the clients you already have and hope that those connections extend out to reach new people.