Center screensaver photo?

[RobertBasil]

Sorry, title should say: Center wallpaper photo?

Is there any script that I can run on our iMacs and Macbooks that will tell the wallpaper to be centered and not stretched?

I know how to set it using a script (and a profile in JAMF) but can't seem to see how to tell it to be centered.

Thanks!

[RobertBasil]

I'm using the following script to change the background:

osascript -e 'tell application "System Events" to set picture of every desktop to ("/Library/Desktop Pictures/wallpaper.jpg" as POSIX file as alias)'