Unfortunately at this point our push API only supports notifying you when a user checks in, which is actually separate from when a photo is added to a check-in, although it seems seamless. Photos are actually added to a check-in via a separate API call (photos/add—you'll notice that there's no way to include a photo in checkins/add itself)
You need to make separate check-in detail calls to see if a particular check-in has a photo associated with it