def image_transform_columns(features):
  """Returns a list of columns that prepare_image_transforms() should run on.

  Because of beam + pickle, IMAGE_URL_TO_VEC_TRANSFORM cannot be used inside of
  a beam function, so we extract the columns prepare_image_transforms() should 
  run on outside of beam.
  """
  import six
  from trainer import feature_transforms

  img_cols = []
  for name, transform in six.iteritems(features):
    if transform['transform'] == feature_transforms.IMAGE_TRANSFORM:
      img_cols.append(name)

  return img_cols