[Django]-How to resize uploaded file with PIL before saving it?

I was able to figure this out. All you need is to save modified file as StringIO and then create a new InMemoryUploadedFile from it. Here is the complete solution:

def save(self):

    import Image as pil
    import StringIO, time, os.path
    from django.core.files.uploadedfile import InMemoryUploadedFile

    # if avatar is uploaded, we need to scale it
    if self.files['avatar']:
        # opening file as PIL instance
        img = pil.open(self.files['avatar'])

        # modifying it
        img.thumbnail((150, 150), pil.ANTIALIAS)

        # saving it to memory
        thumb_io = StringIO.StringIO()
        img.save(thumb_io,  self.files['avatar'].content_type.split('/')[-1].upper())

        # generating name for the new file
        new_file_name = str(self.instance.id) +'_avatar_' +\
                        str(int(time.time())) + \
                        os.path.splitext(self.instance.avatar.name)[1]

        # creating new InMemoryUploadedFile() based on the modified file
        file = InMemoryUploadedFile(thumb_io,
                                    u"avatar", # important to specify field name here
                                    new_file_name,
                                    self.files['avatar'].content_type,
                                    thumb_io.len,
                                    None)

        # and finally, replacing original InMemoryUploadedFile() with modified one
        self.instance.avatar = file

    # now, when form will be saved, it will use the modified file, instead of the original
    super(UserForm, self).save()