@Norm Unless I am misunderstanding how it works the problem appears self evident.
When you click on the thumbnails there is an option in the sidebar panel under the Data section for three separate resolution settings, but when you click on the large image no such option exists.
I've tried removing the large image thinking that it would be controlled by the thumbnails but then nothing appears in the main window when previewed. There is a lack of clarity how this works.
I'm assuming the thumbnail images need to be small, otherwise we are adding greatly to the page size and in theory we could add three variations of thumbnail size for different screen types e.g 50px, 100px and 150px. When it comes to the main image there are no obvious options at all.