I'm not an acoustics expert, but I've treated my workspace and done my research, so I will tell you what I know.
The basic gist is to straddle the main corners in the room with basstraps first. So in your case those would be the 2 corners next to where your speakers currently reside, and the 2 corners behind that door. Think 3D here though: not just the vertical corners count! The horizontal ones across the ceilings and floors make a difference too. With that in mind, the tri-corners (where 2 walls and a ceiling/floor meet) are the most important points of a room, because the bass buildup is gigantic there. Make an assessment of the amount of basstraps you have available, and spread those over the key corners first. If you still have some left after that, you could think about straddling some additional ceiling-wall corners and stuff. It's all going to make a difference!
I don't know much about acoustic foam, and how effective it is, but my first instinct would be to put them in the first reflection points of your monitors from where your listening position is. This is called creating a reflection free zone. The basic idea is to take care of soundreflections that mess with your stereo-image perception. Here is an excellent article to explain what that means: http://www.realtraps.com/rfz.htm
Check that site out anyway, it has some very helpful articles on this entire thing.
I personally went with broadband-absorbers to do this job (basstraps from a material with lesser density, mainly to take care of the mid and high frequencies, because they cause the most problems with your stereo image here).
To add to this, you could also put something in the reflectionpoint between the speakers and you on the ceiling (a so called "cloud") to take care of another early reflectionpoint.
The placement of the monitors and your listeningposition is very important with all of this, so I would suggest starting out by experimenting with a position that works best for that room without compromising your workspace too much. Here's another article from that same site that explains the ideal situation: http://www.realtraps.com/art_room-setup.htm
Since your room is oddly shaped, it is hard to say exactly what the best position will be, but the basics from this article still apply. From the picture, I think your monitor setup vs your listeningposition is already pretty correct. I would try to move it all a bit further away from that back wall though, until the bass starts to sound more natural to you, and with less peaks/holes in the lower frequencies (room modes). Do this BEFORE you start with the reflection free zone, because everything is going to be placed relatively from your listeningposition and the position of your monitors.
I am going to add here that your room looks small/medium sized. This is not to put you off, but keep in mind you may still have some issues in the room, even after treating it. Just mentioning that to keep your expectations realistic. Treatment is still going to be a LOT better than no treatment!
If possible, could you draw a simple top-down map of the room? That would help people to give you more detailed advice I think
And finally, I should mention I'm not in any way connected to the Realtraps company. Their guides are ace though!
