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That's right, jerryB - it can't be a picture of himself.
But I still have to convince brian j john and littleowl.
We all agree that the man looking at the photo is an only child (from clause 1 of his answer).
So everything depends on clause 2 of his answer.
I think we also agree that "my father's son" is "me" -
"me" being the person looking at the picture.
If, when asked who he was looking at, he had replied:
"this man is my father's son", then, right enough, he would have been looking at his own picture ("this man is me").
But he didn't say that. He said "this man's father is my father's son". That is, "the father of this man in the picture is me". Or, "I am the father of the man in the photo, he is my son".
Does that make sense?
Anyway, thanks to all of you for an interesting discussion.