I did upgrade from 16.04 to 18.04, and now no thumbnails, so I must open an image to see what it is! I don't know what the new 18.04 windowmanager/desktopmanager/filemanager exactly are, and which ones are the same with different names,or vice versa! - and which thing would handle thumbnails anyway! I've looked for clues so much that the brain is seized. I've overgoogled it! The current size limit is about 4MB, so its not that... PS: I have bits of kde in the system owing to showfoto install.