Before Bohannon burns the photo, we can see that Corporal Prescott doesn't appear on it. When Bohannon finds the photo in the previous episode we can see that Prescott appears (in fact he starts to remember how he killed him when he sees him on the photo).
The term "black powder" was not used until the invention of smokeless powder in the 1880's. In the time of Hell on Wheels it was known as "gunpowder" or "blasting powder".