Canvas drawImage example 1

function draw() {
  var ctx = document.getElementById('canvas').getContext('2d');
  var img = new Image();
  img.src = 'images/backdrop.png';
  img.onload = function(){
    ctx.drawImage(img,0,0);
    ctx.beginPath();
    ctx.moveTo(30,96);
    ctx.lineTo(70,66);
    ctx.lineTo(103,76);
    ctx.lineTo(170,15);
    ctx.stroke();
  }
}
      

Source image