Step 1: Understanding the Question:
This blood relations puzzle requires us to determine the identity of the person in the photograph based on the speaker's clue.
Step 2: Detailed Explanation:
Analyzing the clue: "I have no brother or sister but that man's father is my father's son."
Because the speaker is an only child, "my father's son" must refer to the speaker.
Replacing this in the original sentence gives: "that man's father is the speaker."
If the speaker is the father of the person in the picture, the person in the picture is undoubtedly the speaker's son.
Step 3: Final Answer:
The correct choice is (B).