如何将上传的图片应用到背景图片中？

<input type="file" id="uploadInput">
<div id="backgroundDiv"></div>

document.getElementById('uploadInput').addEventListener('change', function(event) {
  var file = event.target.files[0];
  var reader = new FileReader();
  reader.onload = function(e) {
    var uploadedImageURL = e.target.result;
    document.getElementById('backgroundDiv').style.backgroundImage = 'url(' + uploadedImageURL + ')';
  };
  reader.readAsDataURL(file);
});

#backgroundDiv {
  width: 500px;
  height: 300px;
  background-size: cover;
  background-position: center;
}