Okay, so here's what I've got so far.
Recognition of a 4 star image is quite good, and a 0 star (in this case a photo I took of art behind glass where the image contained the art and idiosyncratic reflections of the room from a particular angle and me taking the photo) can very rarely hit.
When the image is recognized I am able to tap that part of the screen and am taken to the plaque (etc.) that the AR trigger points to. But within the AR view, the media that has been added to the AR trigger (in the right sidebar) does not replace the recognized image in the video feed. I have tried a jpg and a movie as the attached media.
Being able to, as a person, see something important on the screen and tap it to have the computer know what I'm doing (evidenced by taking me to a new place in ARIS) is really cool. In the long run I think I will enjoy that more than the immediate Harry Potter style overlay. But using the trigger media will still be useful for presenting the player with some confirmation that the image has been identified.
One other note, at first I did not add media to the trigger. When the image is recognized in the app, the app crashes.