That was my preface, but I didn't explain myself. I asked them to tell me what was so significant about the photo.
"Because they're hiding."
"Because they're on a train."
I asked them what the relationship was between those two girls.
"Mother and daughter."
I told them that the photo had rattled me so much because there is only one girl in that photo. Those two girls are the same person, separated by decades. The child grew into the adult who digitally inserted herself back into childhood portraits.
Whether the results constitute "musings on the contemporary relevance of the self-portrait" or, for me, a concise visual metaphor for everything that is so wonderful and horrifying about growing up, or something else entirely, I don't know. But they're wonderful.