tell application "System Events"
tell current desktop
set picture to "/Library/Desktop Pictures/Beach.jpg"
end tell
end tell

tell application "System Events"
    set desktopCount to count of desktops
    repeat with desktopNumber from 1 to desktopCount
        tell desktop desktopNumber
            set picture to "/Library/Desktop Pictures/Beach.jpg"
        end tell
    end repeat
end tell

tell application "System Events"
    tell every desktop
        set picture to "path/to/picture.png"
    end tell
end tell

tell application "System Events" to tell every desktop to set picture to "path/to/picture.png"