changing image path after upload django

from django.db import models

...

class Picture(models.Model):

    ...

    pic = models.ImageField(upload_to='tmp')

    ...

    def save(self, *args, **kwargs):
        #saving the picture first, because it must be uploaded and stored first
        super(Picture, self).save(*args, **kwargs)
        #moving picture if needed
        if blablabla:
            import os
            from django.conf import settings
            path = settings.MEDIA_ROOT + somedirname + '/'
            if not os.path.exists(path):
                os.mkdir(path)
            os.rename(self.pic.path, path + self.pic.name.split('/')[-1])
            #self.pic is ImageFieldFile object
            self.pic = somedir + '/' + self.pic.name.split('/')[-1]
            #self.pic is string object
            super(Picture, self).save(*args, **kwargs)
            #now it's again an ImageFieldFile object with new path