Put it on the market and see if it has value. All kinds of people take photographs of famous people, and most of them don't have much of a market value. If it's a good or interesting photograph by a well known artist, then someone might be willing to pay for it.
Maybe Silverstein was being ironic or making a joke or reacting instinctively to the sort of question I'm sure he gets all the time, and probably finds tiresome. Obviously, it would be impossible to make a hypothetical statement about the market value of an Imogen Cunningham self-portrait that doesn't exist, so I wouldn't take his statement too seriously.