No photos, but from the reading I've done, the correct answer is "whatever they could get their hands on". When you talk from colonial times through the end of the Indian wars, you're covering a lot of territory. I do know the smoothbore trade gun was quite popular through colonial times right up to the Civil War times. Simple, could shoot buck or ball or both, easy to maintain and inexpensive. Even among European settlers, this style of firearm was far more common as a working man's gun than the fine longrifles.
I've seen later photos of native Americans with all sorts of lever actions, spencer carbines, sharps, 1861 Springfields, flintlocks and goodness only knows what. I'm not absolutely sure we could say what was the most common as I think it depends on era and location. I do believe over the years, the smoothbore Trade gun would have to rank up there pretty high as one of the most popular, if not the most popular.