Hi Adam,
Thanks for reaching out!
In my experience, one of the main causes of image quality issues in Storyline 360 is mismatches between the resolution of images being added and the story size of the project. This article explains our best practices to prevent quality loss for images and videos:
If the size of your images isn't too far off from the size of your project, would you be willing to upload a copy of your project file here or in private by opening a support case so we can take a closer look at what's happening? We'll delete it as soon as we're done troubleshooting.