I have another night wall, you've probably missed:
It's simpler to modify.
And specially for you:
The width falls in at 3120 for the max there.. The width you have there would be if you could span over to a 3rd display. Some here are running the tv resolutions of 1920x1080 which would also pose a problem for single displays when the height is lacking. Some were 2720x1080 earlier I had to tweak a bit to meet the 3120x1024 when not left at 1080.
The work is good however. Don't get the wrong impression about that. But I don't know what else I could have done besides shrink to maintain identical aspect and then insert on a smaller 1600x1024 backdrop? That would be a split level paper maybe seeing the lettering above the picture portion on the lower half. Even with a split paper the width was simply too much for the low res for the height. But if you think that put 10 in the Funny Room guess again?!
3840x720 (3xHD) is indeed for 3 displays. (equivalent to 3xFullHD of 5760x1080)
But if one or two displays are missing, the image should be automatically cropped to center view; it was meant to be cropped: no need to squish it all in if the aspect ratio is a fail. If the height is to low, the system should stretch it automatically, if not change wallpaper settings. If you have two screens, it's your choice but by default it should cut the width and stretch the height.
Stretching the image in Photoshop / Paint.NET doesn't do any good either, it makes it easier for your native res; but again native res depends on the user monitor(s).
For one screen I get the centered portion which is all fantastic; 16/9 or 4/3 - 5/4 all look good with height stretch and width crop.