A Good Word

Manhattan private detective Frank Mateo is asked for a favor by a street informant named Dominguez, with whom he has worked for the past ten years. When Dominguez meets an unfortunate end, Mateo believes he (because of the favor he granted) is partially responsible; and so, begins his search for the killer.

However - weeks earlier - Number 49, a painting (by an American artist named Rafique) is returned to its rightful owner, a billionaire from whose home it was stolen several years before.

How is the painting in any way relevant?

Does it have anything to do with the favor Mateo did for Dominguez?

And why was Dominguez, a low-level informant, killed on the street on Halloween night?