Though I like where KEW's head is at... I gotta side with Shady. Even a European Museum with 20 ornate frames per wall... you won't get enough diffraction or absorption to really affect anything in a meaningful way. Especially if there is glass or plexi covering a print. Maybe if you had 2" insulation behind oil-on-canvas paintings.
But seriously... Look at Skyline and Fractal Diffusers if you want to see something that may make a difference on your back wall, and GIK for Absorption panels and/or Absorption-Diffusion panels for front and sides.
Of course... Carpet, boookshelvels, knick-knacks galore, furniture, heavy drapes over windows... These are the laypersons go-to techniques.
Nice link, Shady!!! Thanks. I've got a truckload of 40 minute videos cued up!