How can I link or dynamically update external image files in Storyline 360?

Take a look at my replies to the following post.

Use triggers and variables to cycle through multiple layers in sequence | Articulate - Community

While it does not directly address your issue, it does demonstrate an approach to dynamically reading in and swapping images from a folder that you specify (located in an included web object). The post is a bit older, so it does not take advantage of the newer methods of accessing slide elements with the JS API. 

Nevertheless, you could adapt aspects of this approach to automatically read in and display updated images in your project.

You could also utilize dynamic webURLs as the swapped image sources instead of the static files, if you were so inclined and your hosting sites did not balk about CORS permissions.

You could even access a database to retrieve image data and dynamically swap out the slide image sources. I have another more complicate post related to that, if you are interested.