A Parse beforeSave callback that can be used to check that an uploaded image is not empty.

Post BeforeSave.js

//Change 'Post' to the name of your class
Parse.Cloud.beforeSave('Post', function(request, response) {
  //Change 'file' to the name of the column containing the image
  var imageUrl = request.object.get('file').url();
  Parse.Cloud.httpRequest({
    url: imageUrl
  }).then(function(httpResponse) {
    if (!(httpResponse.buffer && httpResponse.buffer.length)) {
      response.error('Uploaded image was empty or invalid');
    } else {
      response.success();
    }
  }, function (error) {
    response.error('An error occurred checking uploaded image');
  });
});