class ProjectImage(models.Model):
    big_thumb = ThumbnailField(upload_to='profiles', size=(500, 500))
    med_thumb = ThumbnailField(upload_to='profiles', size=(300, 300))
    small_thumb = ThumbnailField(upload_to='profiles', size=(100, 100))

def get_med_thumb_url(self):
    if self.med_thumb is None:
         return self.big_thumb.url

    return self.med_thumb.url