why is the grinning photograph of the father in the spotlight at this point?

The picture represents the idea that father shows no regret for abandoning his family.

"A blown-up photograph of the father hangs on the wall of the living room, to the left of the archway. It is the face of a very handsome young man in a doughboy's First World War cap. He is gallantly smiling, ineluctably smiling, as if to say "I will be smiling forever."