I have troubleshooted by deactivating all plugins, resetting the plugins folder, reverting to the twenty-ten theme, and still no successful result. I found reverting to the twenty-ten theme *resized* images in my gallery to resemble thumbnails, but they’re not using the actual thumbnail files themselves.
When I upload a new image, it creates the thumbnails just fine, and gives me the option to choose from them.
But all the images I uploaded previous to 3.1, thumbnails are broken. The thumbnail files exist in my uploads directory still, but the option to choose them is not present: https://oi56.tinypic.com/1zn0cd1.jpg
Any ideas on how to get those options back, without having to re-upload all those images?