function makeImage(imageUrl, rect, layout) {
  var zrImg = new graphic_Image({
    style: {
      image: imageUrl,
      x: rect.x,
      y: rect.y,
      width: rect.width,
      height: rect.height
    },
    onload: function (img) {
      if (layout === 'center') {
        var boundingRect = {
          width: img.width,
          height: img.height
        };
        zrImg.setStyle(centerGraphic(rect, boundingRect));
      }
    }
  });
  return zrImg;
}
Create a image element from image url
@param imageUrl image url
@param opts options
@param rect constrain rect
@param layout 'center' or 'cover'. Default to be 'cover'