Oh, I see. It only seems to happen with the drag and drop editor, not the themes that come with Newsletter.
So I could just use the Newsletter themes instead — but I have no option to upload images when using the toolbar at the top (when I click to insert/add an image, no upload option appears). The images only upload with the drag and drop for some reason.
I’m using the Divi-based theme “Extra” for my website…maybe it’s causing the problems?