I think that is causing the trouble. If you import a listing from “My eBay” the whole source code of the item description needs to be downloaded. This includes the code for the listing design and the embedded images.
A workaround is to copy the whole text from the item description in Preview mode, switch to Editor mode, clear the item description field there and paste in the copied text. If you then switch back to Preview mode you can add images and a design as usual but need to re-format the text as well.