How to position a button on a scrolled container in flutter

I have several windows that are jpg images of various sizes. For the small ones, I can simply position a "FloatingActionButton" using screen coordinates:

        Positioned(
        height: 150,
        width: 150,
        top: 540,
        left: 860,

How can I position them on large jpg images, where I have to scroll ("SingleChildScrollView") further down the jpg? Increasing the "top" value above the maximum screen coordinate does not do anything.