Tube traps in the corners behind your speakers would be a good place to start. It looks like you have standard wood paneling with suspended ceiling acoustic tile.
Believe it or not some people actually like drywall over some type of insulation. I can't see if you have some type of area rug to prevent floor reflections.
The question is are you having any problems? If not don't worry. Some dealer should be willing to lend you a pair of tube traps to see if it sounds better. If not here is a trick. Stack newspapers as high you can in both corners.
Another trick. Go to home improvement center. Get rolls of unfaced corning insulation. Stack them to the ceiling in the corner. They will take it back.
I would like to hear what others think.