wa alaykum us-Salaam
im not sure how you could do this, but the window could be painted in a way as if you are looking at a house from the outside.
like how some houses have trees/climbers surrounding their windows from the outside. to tie in with that (theme), you could actually paint something on the window as if to create the impression that you can see inside the house itself...like maybe a curtain or something like that. so when you look out the window, the real life outside is inside (as you have the tree on the ceiling) and the real life inside is outside (the curtain or whatever it is you're going to paint on the window)...get me???
I found this tree which I really like but I don't know if it's too much for the kind of ceiling I have or just in general
you could just base your painting on that tree. you dont have to make an exact replica...trees with no color (like the one you posted) are actually really easy to paint as there is no color or shading involved...so you dont have to worry about getting it perfect. you just have to choose the right shade and paint away. but just remember if you are drawing/painting trees, the higher they get, the thinner the branches became (notice the branches in the picture you posted).
EDIT: if it is a small space and the ceiling is that close to the ground, then i suggest you make the elements (eg trees, etc) of your mural small otherwise it will feel too crowded...also, again if the space is small, use light colors as to open the area up...it will feel to claustrophobic then.
i would also suggest you make the window some kind of water feature as water tends to sooth the soul (you probably need that in a small area like that).