Thanks for posting this. This is one of the features of the 4.4 release! It's the Offset From - Float property. https://www.screencast.com/t/ZzuVsRLA8iA
The functionality that we saw is a little
more complex - the image moves a little, and then once you scroll down farther, it doesn't move along with the page. I'm not sure if we can simulate that portion. I think it either floats, or it doesn't.
Also, there is currently a bug in Safari (which, if you're on a Mac, is what is used to preview) where it doesn't float in Preview. We're working on resolving that, but at the moment, if you publish, it should work in other browsers.