Photo wallpaper on the toilet back wall

Wallpaper should actually withstand the occasional wipe with a damp cloth. After all, they are wall coverings - I can also imagine that there are different quality standards like with paint or flooring. I would research that.
I know some who have a patterned wallpaper (with fish) as an accent between the toilet and the washbasin (facing each other) - it looked really great -> Motif: fish on a blue background.

You could paint the poster or wallpaper with elephant skin. It is washable. It has the advantage that you can tear it all down again when you have had enough of it.

Hello everyone,
we have now found something. In case anyone else is interested: There are waterproof vinyl wallpapers (which can even be printed with custom designs). That's what we will take now. A nice beach motif 8-) The price is between €120-140 for 125x245cm.
Alternatively, we were offered to cover a glass wall with a kind of photo foil, but that would have been more expensive. We were advised against putting wallpaper behind a glass wall because waves could form if the wall is not completely flat and/or dust and moisture could accumulate behind the glass.