The system takes the largest single image on a map (by filesize, I believe) and uses that as the page's image. If you remember which image was on the ribbon, is the image still on the map? Is it an image you uploaded or one you found using the Roll20 search? If you found it via search it might be missing from its original location out on the web. You may still see it on your map if it is cached. Also, what is the filetype and extension for the file (e.g., .jpg, .png, etc.)? Roll20 has problems with .gif files and jpgs with the .jpeg extension.
