There are so many other things that could leak besides thumbnails: When you opened that image from an encrypted volume in an image viewer portions of the memor

There are so many other things that could leak besides thumbnails: When you opened that image from an encrypted volume in an image viewer portions of the memory could have been written to swap. All kinds of things will be logged to system log files: what programs you invoked, who knows what else. Opening an encrypted volume on an unencrypted computer is just asking for trouble. If you really need security you should open it on a fresh fully encrypted install of your OS that you dispose after.