how can I make the back wall transparent?

Bass reflections off the wall behind the speakers does not affect perceived image depth. It's the critical midrange where you want to get rid of some reflections.

It all depends on what you call "pleasing" as far as looks.

A few panels of strategically placed Sonex will help immensely.

Some companies make some fabric covered shallow boxes filled with sound absorbent material (fiberglass....) that would do the trick.

Nice Persian carpets, that you so often see in pics of listening rooms, help.

Your photo or a stock photo on a panel.
http://www.acoustimac.com/acoustic-panels/acousticart-panels/

I agree, Tom, I think the op is talking about the front wall.

I'd recommend treating the room with bass traps / absorption panels and diffusors. I'm new to room treatments as I just hung my first four panels (gik 244 broad band traps) but I'm totally sold on room treatment now. I think you can gain what your looking for by treating the first reflection points and going from there!
Good Luck!

not knowing what speakers used u might want to try diffusers instead of using traditional absorbtion...many articles on the net covering this..good luck

I had that issue once and physically moved the speakers out when listening and back when done. It kinda sucked but preferable to hanging weird things on the wall since it was in our living room. So it was worth the effort to me given the huge sonic benefits but maybe not for you. Or maybe there's some product that makes speakers easy to slide without adversely affecting sound?

Other thought was if you may have the flexibility to try the speakers in the corner where the corner itself is the midpoint between the speakers. I've seen manufacturers do this at shows where the room was otherwise too small and seemed to work pretty well. Just a different thought. Best of luck.