Based on those portraits, Peter, I think serenity is one of the key ingredients. She has a well-composed demeanor. Having presence of mind, being present for the eternal present of the photograph--as again she does--, must also form a part of the transaction of dignity. I feel more and more that dignity is a quality composed by both subject and photographer, that it is part of an agreement that is solemn and usually tacit--for otherwise how could we find so many portraits of fellow creatures who are not human but radiate dignity in the gaze they present to this machine we point at them? I'm glad you introduced the term serenity. Dignity has connotations of something to be protected, guarded, or or worn like a mask rather than show scars and doubts and failings. But serenity radiates outward from spiritual calm.
(...She also has eyes like searchlights. I'd certainly need my dignity if they turned on me to appraise my character.)