WallPaper set for MAC OSX via Bigfix

Can someone help me with script to update New wallpaper on MAC Book via Bigfix .
I tried using osascript , when i try locally it works but when i try via bigfix using current user it does not.

following is the script i used : osascript -e ‘tell application “Finder” to set desktop picture to POSIX file “/tmp/Wallpaper.jpg”’

i tried this

parameter “CurrentUserName” = "{unique value of names of (current users; logged on users)}"
run su {parameter “CurrentUserName”} -c “osascript -e ‘tell application “Finder” to set desktop picture to POSIX file “/tmp/WallPaper.jpg”’”

This is not working for me.
Any help will be appreciated !!!