OP asks about having the same background on both monitors with the desktop slideshow. As of Windows 8 and newer (including Windows 10), that's not possible unless you use the Tile or Span fit...

This is correct. The auto-sync of themes limits their maximum size. If you want a "full size" theme, you will need to manually sync/copy it across machines.

You can use the old interface by running exactly:

control /name Microsoft.Personalization /page pageWallpaper

from a command prompt. Does it work there... ?

Are you using multiple machines? Might want to turn off theme roaming if so in the Settings application under Account Settings under Other options or so.

Is this a JPEG? JPEGs get imported somewhat lossily. You can override the "JPEGImportQuality" to fix that.

Are you trying to use subfolders? Those generally aren't well-supported.

It's just over in the Settings application in Personalization :: Background.

Nope, you're right. That switch is safe. I should've followed the link: I was thinking it was the trick theme rather than just moving to aerolite. Keep up the good work

edit: nope, that one actually is solid.

10565 updates the Styles files (in order to provide the new white assets used for darker themes). Since people using the colorized title bar trick theme wouldn't have those updates, those people's...

Have you tried this with the latest Insider build? 10565 might be of high interest to you.

You can use:

control /name Microsoft.Personalization /page pageWallpaper

from a Run or CMD prompt to do this for now. Note it generally only works for the Pictures library.

Naw, OP is just confused and is (ab)using) Slideshow to set Single Picture mode. Cyberghost's post here:

*...

There's some weird video card driver going around that is breaking multimon wallpaper. If you're using multimon and don't have the "set for monitor X" options, your video card driver is broken, I...

If you want to set a distinct wallpaper per monitor without shuffling, go to:

control /name Microsoft.Personalization /page pageWallpaper

from a CMD or Run prompt. Then when you're there...

Looks like it's baked into the theme, and was only centered for Windows 8.

When you're using the colorized version of the Windows 10 theme, what don't you look about it?

Can you just directly run:

control mouse

from a CMD or Run prompt?

... Has there ever been a way to do this in Windows? Maybe I'm dumb and missed this option.

edit: Looks like this is left-aligned in Windows 7, center-aligned in Windows 7, and left-aligned again...

What happens if you use the "Tile" option in the "Choose a fit" dropdown on the Personalization page?

Looks like there's a cap on those values that prevents it from being outside of certain ranges. :\

Nanodice: does using the control panel method I mentioned work for me? Seems like an easier route. :)

Fair. It's not really rocket science, though.

https://www.tenforums.com/tutorials/9108-sign-screen-background-image-enable-disable-windows-10-a.html

Run:
reg query...

You need to be on 10240 with the latest updates. It's not in vanilla 10240.

After the latest updates, looks like you can set DisableLogonBackgroundImage to REG_DWORD value 0x1 under HKEY_LOCAL_MACHINE\Software\Policies\Microsoft\Windows\System to turn this off. If you want...