The picture almost certainly has been flipped. That is usually attributed to a reversed negative, but I have recently learned that reversal is a result of the old time photographic processes, which didn't use negatives as such. That seems to be the explanation for the famous "left handed" Billy the Kid photo and probably the same is true with this picture which could well be from the same or an earlier era.
Assuming the picture is not a modern fake, the horn would seem to indicate a muzzle loader. The hammer doesn't look like a mule-ear. It is a back action lock, uncommon on American guns, and appears to be a percussion rifle.
Knowing something about the source of the picture would help, but I doubt anyone will ever identify the gun in any detail from that photograph alone.
Jim