I don't think I've ever seen good versions of those floating around.
I remembered that there are alternate versions in the artbook extra that comes w/ the Divine Edition. But looking at it again, all of those are rectangular crops of the in-game textures. What I mean is all of the textures are odd shapes, made to fit the panels of the menu triptych. And for a book or a wallpaper, you want some kinda rectangle.A picture to explain.
The left half is from the artbook, the right half is a scaled version of the game texture, and the blue outlines show roughly the same crop. I would guess full resolution "rectangular versions" don't exist, and these odd shapes are the originals.
But you could take the same approach & make your own wallpapers w/ the game textures. They're pretty high res.
Even with the taller 'side panels' the thin upper section is ~2100px wide, so you could cut out at least a 1920x1080 wallpaper.