I fell into the water at Fossil Falls and underneath the waterfall I found a cave with an empty picture frame at the end. The hat on my had asked what the purpose of the picture frame was, but it didn’t do anything else to give a clues on what this item actually does and how to activate it. What are you meant to do to activate the blank picture frame beneath the waterfall of Fossil Falls?
They usually get triggered once you complete certain events in the area. There are lots of picture frames all over the game. Once they activate, they will allow you to teleport to a location in a different world. I don’t know what triggers the picture frame beneath the waterfall in Fossil Falls to activate, but there is likely some activity or moon in the area that you need to obtain and it will activate then, or maybe you have to progress with the story a bit first.