Tor and Tails merge to support western intelligence

For images, you can use a photo library application like digikam and set tags on the files, which are saved both to a relational database and to the photo metadata inside the image file. For other file types I don’t think there’s anything standard.

I think it might store tags in linux file system xattrs so other software (or scripts) could access and index it.

I guess it switched desktop environments on you? If you’re logged out there’s supposed to be a way you can switch between desktop environments. It makes sense that the GNOME Settings app would only change wallpaper settings for GNOME, which I think is the main Ubuntu desktop environment. Are you sure you didn’t upgrade to a version of Ubuntu that uses XFCE instead of GNOME?