F. Alton Everest has a book called "Acoustics" ( I think that's the title) that is the bible in this area. It can be had thru Mix Magazine bookshelf bookstore on the web. I don't know the URL but I'm sure a search will find it. It will give you much more knowledge than you ever wanted on the subject

Generally, keep the room irregular. Clap your hands as you walk around the room, and if you hear any slap echoes, put something on one or both of the walls in that area until the slap is gone. You don't want a completely dead room, nor a really live one. A reverb period of about 1 second or less is good. Put the most acoustic treatment near the front speakers on the walls and floors to alleviate reflections. Near the surrounds, you might want to keep it somewhat live, to enhance the sense of envelopment from those speakers. By looking at your setup, I think you are well on your way to the ideal acoustic surroundings. It looks like your room might be on the live side, which is good. If that is the case, you can put hangings on the wall and throw carpets in strategic locations to tame the room to your liking.

Have fun!