First off, this is 100% an annoyance that just happens to be bugging me, who may or may not have some degree of OCD! ;-)
How do you tell Windows 10 to begin tiling a wallpaper on the display that is set as the main display, instead of the display that just happens to be the one on the left? Read on if you want to see the too-much-information description.
I have 3 screens: My laptop which is open and used as an extra display is listed as display 1 in Windows. I also have two 24" monitors connected to the USB dock that is attached to my laptop, which are listed as displays 3 and 2. Display 3 is set as the main (*) display. They are arranged on my desk as such:
[ 1 ] [*3*] [ 2 ]
Resolutions are set for 1920x1080 on the monitors (2 and 3) and 1600x900 on the laptop (1).
Now here's the thing - I was using a wallpaper that is 3840x1080 set to "tile". This would completely fill the 24" monitors. In Win7, the wallpaper's bottom left corner would start in the bottom left of display 3, so the two monitors had the whole wallpaper, and display 1, the laptop, would have a portion of the wallpaper repeated on it. This was fine.
Now that I've upgraded to Win10, the displays are set up in the same way, but the wallpaper is "starting" on display 1 instead of 3 (which is still set as the main display.)