Comment 17 for bug 1269992

syscon-hh: My guess is that this is due to:

2013-10-28 Vladimir Serbinenko <email address hidden>

        Make / in btrfs refer to real root, not the default volume.
        Modify mkrelpath to work even if device is mounted with subvolid option.

... so I think the question is why the "/@" prefix is still there.
Could you confirm the output of this command, please?

  sudo grub-mkrelpath /usr/share/backgrounds/gnome/Stones.jpg