You can find the packages containing the Ubuntu wallpapers e.g. here: https://old-releases.ubuntu.com/ubuntu/pool/main/u/ubuntu-wallpapers/

Looking at the origin:
ambi, Greek: both;
anti, Greek: against, opposed;
dexter, Latin: right, skilful, clever;
sinister, Latin: left, wrong, evil;

So sinister is already anti-dexter, the ambi just emphasises that this not-skilfulness applies to both hands. In German, calling somebody having “two left hands” means that they aren’t skilful at all concerning handcrafting.