var delay = 4000;
      var ImgNum = 0;
var ImgLength = 0;

var lock = false;
var run;

function chgImg(direction) {
  if (document.images) {
    ImgNum = ImgNum + 1;
  }
  if (ImgNum > ImgLength) {
  ImgNum = 0;
  }


  document.getElementById('mainpic').style.backgroundImage = 'url(' + NewImg[ImgNum] + ')';
}

function auto() {
  ImgLength = NewImg.length - 1;
  if (lock == true) {
    lock = false;
    window.clearInterval(run);
  }
  else if (lock == false) {
    lock = true;
    run = setInterval("chgImg(1)", delay);
  }
}