Many ways to do it. Here is the cheapest, then my way lol.
Look for posters on moviepostersdb.com. At least min of 150dpi.
Print at SpoonFlower.
Look for 1" stretch bars online, I think they range at $6+.
Mount the pictures yourself on the stretch bars.
then your acoustic fiber board.
Here was what I did.
Bought movie posters from movieposters.com. $16+
took them to local scanner guy who can scan large image $30 a piece
Print at Spoonflower
I took them to local framing guy $55 for small up to $95 for the large.
Then the acoustic 5lb 1" fiber board. 12 * 2x4 = $100
Spoonflower.com is where I went to get the images printed.
Their first material, I think it's called cotton quilt. $18 yard is the best for allowing sound waves to pass through. This was tested to be as good as anything out there, but still offered enough material to give a decent image.
The small ones took 1yard each, whereas the larger ones took 2 yards, those on the side walls.
Maximum is 42" for width, No max length.