Some background first. I have three 1920x1080 monitors. The central monitor is Landscape. The two side monitors are portrait, used for working with web browsers, code, communication . . . I find it easer to deal with large amounts of text in portrait, where I can continuously track a lot of text without tracking back and forth across the entire width of a widescreen monitor.
So, I'm trying to set a background. It works fine. It's abstract-ish, so I set the 'stretch' option. That works fine on the central monitor.
On the two side monitors, the image is compressed between the left and right sides of the screen, but does not stretch to reach the top or bottom of the screen. The extent that they seem to extend up and down seem to exactly match the dimensions and alignment of the central screen. If this is so, then it would mean that on the portrait monitors, the background is filling a 1080x1080 block in the center of a 1080x1920 display.
The only thing that functions properly is 'Tile'. None of the other options seem to produce any change in the image displayed. (In some of the options, though not all, this would be normal.)