I live in a rental property that has a tiny yard out back. I’d love to paint it just to brighten it up a little more for sitting out in the 3 days of UK sun, but don’t want the landlord to take a hissy fit and steal my deposit. So if I did paint it, is there a (easy) way to strip it before I leave? 
You could get some pretty thin sheet rock or maybe play wood, lay them on the ground on some cheap painters plastic sheeting then paint a mural or whatever you want, then when dry you could mount it directly to the wall. Just unscrew it when you leave and voila.
I think because of all the stones it would be hard to screw anything to it. Gave up on the idea of mounting a bike rack thing because of it. But if you any tips on how to get around that hen I’m all ears!
Idk how well this would work for this use, but you could look into using 3M VHB tape (vhb stands for very heavily bonding).
It works similar to those pull release command hooks - same type of tape, but there’s a foam layer to account for uneven surfaces. So when you are ready to unmount it, you pull the tape to stretch it out and it loses adhesion.
It’s used a lot for mounting signs to buildings/glass (often with silicone as well, because that’s more permanent).
If you want to try it, I would suggest testing an area near the bottom and let the tape sit in and set up for at least a week before you pull to remove and see what happens with it.
I’ve used it on interior walls with no issues (great for hanging mirrors and stuff) my only concern would be the texture on the wall preventing a proper pull.