Martingales aren't allowed for a flat class, but they are for over fences. The photo above wasn't taken in a flat class, but most likely when they were entering or leaving the ring for an over fences trip. It's very, very hard to find a photo of a USEF hunter in an under saddle class. If you do a Google image search, 90% of what comes up are QHs and Arabs, which are judged differently and have very different frames.
This is a slightly better example; but IMO if it was a horse rather than pony, the head and neck would not appear quite so stretched out. Somewhere between this pony and the first horse is where you want to be.