I'm not certain, but I believe there were thousands of such portraits (complete with peace symbol) made of Imogen by aspiring young photographers and students. Imogen was very accessible, especially during the time of which you speak. Unfortunately it is rarely the portraits of photographers that have value to collectors, even when these portraits are made by other well-known photographers. It is the works they create that have the value to those who invest in and collect photographs. I'm sure though that someone will find value in your portraits and I suggest you place them on eBay for instance. You will see then what they might be worth to someone.
Best of Luck,