How to set positioning/filling of desktop background image with AppleScript
Solution 1:
According to
'AppleScriptEditor' > File > Open Dictionary >
Finder > Legacy Suite > Application > 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.