Tuba, like most GTK apps, use GdkPixbuf to handle images. While it can handle most basic formats on its own, some newer formats require external loaders. Loader package names are usually <format>-pixbuf-loader.

As packaged in nixpkgs, Tuba is not packaged with the webp-pixbuf-loader being available to it. To make it so, change your configuration to install Tuba with an override like the one below:

(pkgs.tuba.overrideAttrs (old: {
  postInstall = ''
    export GDK_PIXBUF_MODULE_FILE="${gnome._gdkPixbufCacheBuilder_DO_NOT_USE {
      extraLoaders = [
        pkgs.webp-pixbuf-loader
      ];
    }}"
  '';
}))

The flatpak includes the webp-pixbuf-loader. As far as I know, no backends use any other of the newer formats so no other loaders are included. If you notice any, do let us know!