Electrostatics, Rear walls, and Art

My present set-up has Acoustat 1+1 electrostatic speakers in front of bare plaster walls. Generally, I'm pleased with the sound. However, I'm thinking about hanging art -- mostly photographs behind glass -- on the rear wall. Being that the walls are now bare I don't think glass picture frames would make much difference, or would it?
Also, what about some paintings (no glass) or should I leave them on the side walls(in front of the speakers in the listening area) where they are now ?
I need the opinions of all you knowledgeable Audiogoners. Thanks!
I would expect glass to significantly increase the reflections you'll be getting off the rear wall. The paintings may actually diffuse the sound if they are oils with lots of texture - good at the first reflection point.

It should be a relatively easy thing to try any given setup and listen. Your ears in your room is what counts.

Glass behind the speakers is generally not a good idea. Not only will it add to high frequency reflections, but glass will also resonate, unless it is a laminated acrylic on glass. As to the oils--they could work nicely there, but it sounds like they are getting rid of the first order reflection points. You need something to absorb a bit on the side walls, so if you move them you will likely have to add a suitable replacement.
If you are putting art work behind your speakers because you like the way it looks, the ideas already presented are on target.

If your intention is for the art to look nice as well as improve acoustics, there is a better solution for your problem.

Build a frame around rigid fiberglass (acoustic) paneling manufactured by Owens Corning or Johns Manville, then cover with decorative material that is acoustically transparent.

You have attractive panels and effective treatment of the back wave of your Acoustats.

Here is a link to a site with information about various Owens Corning materials.

Coverings for these is limited only by your imagination.