I'm glad that helped, Michael! As for the exact dimensions, the image in this example is from Content Library 360 and was cropped within the app. See image:
https://drive.google.com/file/d/1yOw-_0HrMz-JwXnQjV2PA0egMPniw_-A/view?usp=sharing So, I don’t know the exact dimensions.
In Rise 360, there's no aspect ratio rule to follow since the image will display differently depending on the device used. When you preview your course, you can click the device icons at the top to test how it will look on different devices to make sure it looks right. It sounds like you might already be doing that and just not getting the results you’re looking for. You might need to use a different image if that's the case. But just in case, do you mind sharing a link to your project so I can take a look? I can’t promise anything, but I hope we can find a solution.