While it is most definitely difficult to tell for sure whether it is Marie or Olga in that picture, I think it's Marie for two reasons: 1) her and Anastasia are wearing matching outfits. It appears in most photographs that as the girls got older, they broke off into the "Little" and "Big" pairs in terms of coordinating outfits. I could be wrong, but usually Tatiana and Olga would wear something similar to the two younger girls, but generally not the same. 2) the girl in the picture has a slightly rounder face. Marie is known to have that cherubic type of face. Also, if you look at other pictures of Marie with her hair up, she looks like the girl in the picture.