When identifying individuals in a photograph, use their complete name (as best as you have it including name name) and years of birth and death (if you have it). If you want to include relationships in the identification, do that with respect to other people in the photograph and not to someone not in the photograph.
It can be confusing to see individuals listed as gg-grandpa of MJN, gg-aunt of MJN, 3rd cousin of MJN, etc. Personally I would leave out the relationships unless it helped to clarify who someone was in the photograph.
And don’t forget the location and when the photograph was taken–if you know it. Including who made the identification is helpful as well.