How to set positioning/filling of desktop background image with AppleScript
According to

'AppleScriptEditor' &gt; File &gt; Open Dictionary &gt;
   Finder &gt; Legacy Suite &gt; Application &gt; desktop picture


all you can do is set the picture.

There are references to messing about with the .plist file, but the internet (and Apple) are silent on an Applescript-approved route.

This plist is empty for me in Sierra:  ~/Library/Preferences/com.apple.desktop.plist so I imagine Apple have hidden what used to be there inside one of their even less well documented databases.