set all desktops

set spacenumber to 9
tell application "System Events"
	set _desktops to every desktop
	
	if (count of _desktops) > 1 then
		-- if you have multiple displays (eg: external monitor) ask which
		display dialog "Choose your screen." buttons (get display name of every desktop) default button 1
		set _display to the button returned of the result
	else
		-- else set the one display as selected
		set _display to display name of the first desktop
	end if
	
	set new_file to choose file with prompt "Choose an image."
	repeat with i from 0 to (spacenumber - 1)
		delay 1
		tell (every desktop whose display name is _display) to set picture to new_file as alias
		tell process "WindowServer" to key code {124} using {control down, command down} -- Go to the next space
	end repeat
end tell