I though of that when I was looking for a new flat. Got one with white walls and was looking for something to put on them.
Went to the shop and found a few things but then realized that I could do one myself...
The techniques are very simples but the result look great (well I think). This works with trees but also with old ruins, lonely landscape.
Here is what the result look like:

Here is the link:
Calming Tree Poster Tutorial
Hope you guys like it