10 hours ago
If you use the Thumb search, and choose the "All Languages" checkbox it appears Jellyfin (pre 10.11) is now believing the backdrops are horizontal poster images because of the language tagging. Now if you choose one it's going to use it as a Thumb image of course. But you can still use this method to browse the images at least. Middle-click one if you want to open it in a new tab, or right-click and Save Link As to save it locally, and then you can upload it manually to the server with the + button.