code: def tostring(im, string_format, **options):
      out = BytesIO()
      im.save(out, string_format, **options)
      return out.getvalue()
  
imports: import logging
  import os
  import shutil
  import sys
  import sysconfig
  import tempfile
  from io import BytesIO
  import pytest
  from packaging.version import parse
  from PIL import Image, ImageMath, features
label_desc: Use image node_id 15 for calling a global function with example usage: tostring(im, string_format) and returns: out