To have an enduser add or edit images used as multiple backgrounds for use in a backstretch script you can use the ACF Gallery field for the user to add the images and then use a foreach loop to add the images to the backstretch init script.
Set up the gallery field and link it to your page.
In the page add the images via the new gallery field
Then enqueue the main backstretch.min.js, then for the init script you can add that directly to your WordPress page template using the foreach php loop inside the jQuery script. So the php runs on the server before the jQuery script is rendered in the browser.