I think you may be referring to the Royal Family Order. These are worn by female members of the Royal Family at state occasions and are the gift of the reigning monarch. Camilla, HRH The Duchess of Cornwall was awarded the Royal Family Order of Elizabeth II within the last few years.

The order is merely decorative, unlike military orders male members of the Royal Family may wear at state occasions. They consist of an encrusted portrait of the monarch, hanging from a ribbon as you say, worn on the left hand side.

Can someone add some pictures, for the laymen among us to see what you are talking about? :)