Room Treatment? How important is it to treat the wall behind/between the speakers?

Hi all,

I've treated first reflections on the side walls and some bass absorption with 2 GIK Sound Blocks on the side walls next to each speaker - which seemed to work better than directly behind them.

The picture behind the speakers is painted canvas (reflective) but stuffed with some leftover Rockwool - which I understand is probably not doing much.

So my question is, should the painting be replaced with something that is effective next and if so, what should I use?

Pics in my virtual system.

Thank you.