Hi Jim,
Thanks for sharing a copy of your project file. It looks like advanced HTML5 attributes are needed to properly scale the image down, and how the image is added as a web object in a Storyline 360 course doesn't allow for such advanced attributes to be added.
Can I ask if you've considered adding the image directly to your Storyline 360 slide instead? This will allow you to fully control the appearance of the image on the slide, and since your files are already hosted on Amazon S3, downloading the image files can be done in just a few clicks.