Minnie Thackeray by Julia Margaret Cameron, 1865
I believe Minnie Thackeray must be a reference to Harriet Marian Thackeray, the daughter of the novelist William Makepeace Thackeray, sister to novelist Anne Isabella Thackeray Ritchie, and the first wife of Leslie Stephen (who after her death from childbirth married Julia Jackson). See my other post here for more info. I believe this photo was taken before her marriage.
source: MET Museum