Diffusion could help your sound if placed on the wall behind your chair. Although I doubt it, it might even help on the side walls as well.
Diffusion on side walls will help in soundstaging and depth. However, it will depend on how close your speakers are to the side walls. For small rooms with the speakers close to side walls, the effect will be larger. In your case whereby the side walls are further away from the speakers, the effect may not be too substantial though. Anyway you can't place anything on your left wall since there is a door there.
The standard reply from Ethan is to place absorption at first and 2nd reflection points and diffusion everywhere else. I remember reading this somewhere in the forums or his website.