What does Robicheaux find suspicious about the photograph he finds in Chapter 23?

Robicheaux travels to Durango, Colorado to investigate Lila's cousin's death. Here he finds an old photograph of Archer Terrebonne, Harpo Scruggs and a few other men taken shortly after the cousin's death. Robicheaux remembers that Archer Terrebonne claimed to know nothing about Harpo Scruggs.