SwiftWallpaper.swift

//: Playground - noun: a place where people can play

import Cocoa

//path to image
let image = "/Library/Desktop Pictures/Sierra.jpg"
let url = NSURL.fileURL(withPath: image)

let workspace = NSWorkspace.shared()
let screen = NSScreen.main()
let dic: [String : Any] = ["NSWorkspaceDesktopImageScalingKey" : 1]
var result: () = try workspace.setDesktopImageURL(url, for: screen!, options: dic)

Swift Playground code to change the wallpaper using Swift 3.